Add /config dir support

This commit adds support for a configuration directory (default `config`). The different pieces in this puzzle are:

* A new `--environment` (or `-e`) flag. This can also be set with the `HUGO_ENVIRONMENT` OS environment variable. The value for `environment` defaults to `production` when running `hugo` and `development` when running `hugo server`. You can set it to any value you want (e.g. `hugo server -e "Sensible Environment"`), but as it is used to load configuration from the file system, the letter case may be important. You can get this value in your templates with `{{ hugo.Environment }}`.
* A new `--configDir` flag (defaults to `config` below your project). This can also be set with `HUGO_CONFIGDIR` OS environment variable.

If the `configDir` exists, the configuration files will be read and merged on top of each other from left to right; the right-most value will win on duplicates.

Given the example tree below:

If `environment` is `production`, the left-most `config.toml` would be the one directly below the project (this can now be omitted if you want), and then `_default/config.toml` and finally `production/config.toml`. And since these will be merged, you can just provide the environment specific configuration setting in you production config, e.g. `enableGitInfo = true`. The order within the directories will be lexical (`config.toml` and then `params.toml`).

```bash
config
├── _default
│   ├── config.toml
│   ├── languages.toml
│   ├── menus
│   │   ├── menus.en.toml
│   │   └── menus.zh.toml
│   └── params.toml
├── development
│   └── params.toml
└── production
    ├── config.toml
    └── params.toml
```

Some configuration maps support the language code in the filename (e.g. `menus.en.toml`): `menus` (`menu` also works) and `params`.

Also note that the only folders with "a meaning" in the above listing is the top level directories below `config`. The `menus` sub folder is just added for better organization.

We use `TOML` in the example above, but Hugo also supports `JSON` and `YAML` as configuration formats. These can be mixed.

Fixes #5422

1 files changed, 6 insertions, 2 deletions

diff --git a/common/herrors/error_locator.go b/common/herrors/error_locator.go
index 88cb06c8c..3a72f4790 100644
--- a/common/herrors/error_locator.go
+++ b/common/herrors/error_locator.go
@@ -17,10 +17,10 @@ package herrors
 import (
 	"io"
 	"io/ioutil"
+	"path/filepath"
 	"strings"
 
 	"github.com/gohugoio/hugo/common/text"
-	"github.com/gohugoio/hugo/helpers"
 
 	"github.com/spf13/afero"
 )
@@ -172,12 +172,16 @@ func chromaLexerFromType(fileType string) string {
 	return fileType
 }
 
+func extNoDelimiter(filename string) string {
+	return strings.TrimPrefix(".", filepath.Ext(filename))
+}
+
 func chromaLexerFromFilename(filename string) string {
 	if strings.Contains(filename, "layouts") {
 		return "go-html-template"
 	}
 
-	ext := helpers.ExtNoDelimiter(filename)
+	ext := extNoDelimiter(filename)
 	return chromaLexerFromType(ext)
 }