Add support for URLs relative to context root

Setting `RelativeURLs` to `true` will make all relative URLs in the site *really* relative.

And will do so with speed.

So:

In `/post/myblogpost.html`:

`/mycss.css` becomes `../mycss.css`

The same in `/index.html` will become:

`./mycss.css` etc.

Note that absolute URLs will not be touched (either external resources, or URLs constructed with `BaseURL`).

The speediness is about the same as before:

```
benchmark                    old ns/op     new ns/op     delta
BenchmarkAbsURL              17462         18164         +4.02%
BenchmarkAbsURLSrcset        18842         19632         +4.19%
BenchmarkXMLAbsURLSrcset     18643         19313         +3.59%
BenchmarkXMLAbsURL           9283          9656          +4.02%

benchmark                    old allocs     new allocs     delta
BenchmarkAbsURL              24             28             +16.67%
BenchmarkAbsURLSrcset        29             32             +10.34%
BenchmarkXMLAbsURLSrcset     27             30             +11.11%
BenchmarkXMLAbsURL           12             14             +16.67%

benchmark                    old bytes     new bytes     delta
BenchmarkAbsURL              3154          3404          +7.93%
BenchmarkAbsURLSrcset        2376          2573          +8.29%
BenchmarkXMLAbsURLSrcset     2569          2763          +7.55%
BenchmarkXMLAbsURL           1888          1998          +5.83%

```

Fixes #1104
Fixes #622
Fixes #937
Fixes #157

1 files changed, 8 insertions, 2 deletions

diff --git a/transform/chain.go b/transform/chain.go
index ab26a350e..4019dca25 100644
--- a/transform/chain.go
+++ b/transform/chain.go
@@ -23,6 +23,7 @@ func NewEmptyTransforms() []link {
 // contentTransformer is an interface that enables rotation  of pooled buffers
 // in the transformer chain.
 type contentTransformer interface {
+	Path() []byte
 	Content() []byte
 	io.Writer
 }
@@ -30,10 +31,15 @@ type contentTransformer interface {
 // Implements contentTransformer
 // Content is read from the from-buffer and rewritten to to the to-buffer.
 type fromToBuffer struct {
+	path []byte
 	from *bytes.Buffer
 	to   *bytes.Buffer
 }
 
+func (ft fromToBuffer) Path() []byte {
+	return ft.path
+}
+
 func (ft fromToBuffer) Write(p []byte) (n int, err error) {
 	return ft.to.Write(p)
 }
@@ -42,7 +48,7 @@ func (ft fromToBuffer) Content() []byte {
 	return ft.from.Bytes()
 }
 
-func (c *chain) Apply(w io.Writer, r io.Reader) error {
+func (c *chain) Apply(w io.Writer, r io.Reader, p []byte) error {
 
 	b1 := bp.GetBuffer()
 	defer bp.PutBuffer(b1)
@@ -57,7 +63,7 @@ func (c *chain) Apply(w io.Writer, r io.Reader) error {
 	b2 := bp.GetBuffer()
 	defer bp.PutBuffer(b2)
 
-	fb := &fromToBuffer{from: b1, to: b2}
+	fb := &fromToBuffer{path: p, from: b1, to: b2}
 
 	for i, tr := range *c {
 		if i > 0 {