:sparkles: Implement Page bundling and image handling

This commit is not the smallest in Hugo's history.

Some hightlights include:

* Page bundles (for complete articles, keeping images and content together etc.).
* Bundled images can be processed in as many versions/sizes as you need with the three methods `Resize`, `Fill` and `Fit`.
* Processed images are cached inside `resources/_gen/images` (default) in your project.
* Symbolic links (both files and dirs) are now allowed anywhere inside /content
* A new table based build summary
* The "Total in nn ms" now reports the total including the handling of the files inside /static. So if it now reports more than you're used to, it is just **more real** and probably faster than before (see below).

A site building  benchmark run compared to `v0.31.1` shows that this should be slightly faster and use less memory:

```bash
▶ ./benchSite.sh "TOML,num_langs=.*,num_root_sections=5,num_pages=(500|1000),tags_per_page=5,shortcodes,render"

benchmark                                                                                                         old ns/op     new ns/op     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      101785785     78067944      -23.30%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     185481057     149159919     -19.58%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      103149918     85679409      -16.94%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     203515478     169208775     -16.86%

benchmark                                                                                                         old allocs     new allocs     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      532464         391539         -26.47%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     1056549        772702         -26.87%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      555974         406630         -26.86%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     1086545        789922         -27.30%

benchmark                                                                                                         old bytes     new bytes     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      53243246      43598155      -18.12%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     105811617     86087116      -18.64%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      54558852      44545097      -18.35%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     106903858     86978413      -18.64%
```

Fixes #3651
Closes #3158
Fixes #1014
Closes #2021
Fixes #1240
Updates #3757

1 files changed, 116 insertions, 0 deletions

diff --git a/helpers/processing_stats.go b/helpers/processing_stats.go
new file mode 100644
index 000000000..36dfcda69
--- /dev/null
+++ b/helpers/processing_stats.go
@@ -0,0 +1,116 @@
+// Copyright 2017 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package helpers
+
+import (
+	"io"
+	"strconv"
+	"sync/atomic"
+
+	"github.com/olekukonko/tablewriter"
+)
+
+type ProcessingStats struct {
+	Name string
+
+	Pages           uint64
+	PaginatorPages  uint64
+	Static          uint64
+	ProcessedImages uint64
+	Files           uint64
+	Aliases         uint64
+	Sitemaps        uint64
+	Cleaned         uint64
+}
+
+type processingStatsTitleVal struct {
+	name string
+	val  uint64
+}
+
+func (s *ProcessingStats) toVals() []processingStatsTitleVal {
+	return []processingStatsTitleVal{
+		processingStatsTitleVal{"Pages", s.Pages},
+		processingStatsTitleVal{"Paginator pages", s.PaginatorPages},
+		processingStatsTitleVal{"Non-page files", s.Files},
+		processingStatsTitleVal{"Static files", s.Static},
+		processingStatsTitleVal{"Processed images", s.ProcessedImages},
+		processingStatsTitleVal{"Aliases", s.Aliases},
+		processingStatsTitleVal{"Sitemaps", s.Sitemaps},
+		processingStatsTitleVal{"Cleaned", s.Cleaned},
+	}
+}
+
+func NewProcessingStats(name string) *ProcessingStats {
+	return &ProcessingStats{Name: name}
+}
+
+func (s *ProcessingStats) Incr(counter *uint64) {
+	atomic.AddUint64(counter, 1)
+}
+
+func (s *ProcessingStats) Add(counter *uint64, amount int) {
+	atomic.AddUint64(counter, uint64(amount))
+}
+
+func (s *ProcessingStats) Table(w io.Writer) {
+	titleVals := s.toVals()
+	data := make([][]string, len(titleVals))
+	for i, tv := range titleVals {
+		data[i] = []string{tv.name, strconv.Itoa(int(tv.val))}
+	}
+
+	table := tablewriter.NewWriter(w)
+
+	table.AppendBulk(data)
+	table.SetHeader([]string{"", s.Name})
+	table.SetBorder(false)
+	table.Render()
+
+}
+
+func ProcessingStatsTable(w io.Writer, stats ...*ProcessingStats) {
+	names := make([]string, len(stats)+1)
+
+	var data [][]string
+
+	for i := 0; i < len(stats); i++ {
+		stat := stats[i]
+		names[i+1] = stat.Name
+
+		titleVals := stat.toVals()
+
+		if i == 0 {
+			data = make([][]string, len(titleVals))
+		}
+
+		for j, tv := range titleVals {
+			if i == 0 {
+				data[j] = []string{tv.name, strconv.Itoa(int(tv.val))}
+			} else {
+				data[j] = append(data[j], strconv.Itoa(int(tv.val)))
+			}
+
+		}
+
+	}
+
+	table := tablewriter.NewWriter(w)
+
+	table.AppendBulk(data)
+	table.SetHeader(names)
+	table.SetBorder(false)
+	table.Render()
+
+}