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

common/hugo/hugo.go

@@ -18,28 +18,50 @@ import (
 	"html/template"
 )
 
-var (
-	// CommitHash contains the current Git revision. Use make to build to make
-	// sure this gets set.
-	CommitHash string
+const (
+	EnvironmentDevelopment = "development"
+	EnvironmentProduction  = "production"
+)
 
-	// BuildDate contains the date of the current build.
-	BuildDate string
+var (
+	// commitHash contains the current Git revision. Use make to build to make
+	// sure this gets set.
+	commitHash string
+
+	// buildDate contains the date of the current build.
+	buildDate string
 )
 
 // Info contains information about the current Hugo environment
 type Info struct {
-	Version    VersionString
-	Generator  template.HTML
 	CommitHash string
 	BuildDate  string
+
+	// The build environment.
+	// Defaults are "production" (hugo) and "development" (hugo server).
+	// This can also be set by the user.
+	// It can be any string, but it will be all lower case.
+	Environment string
 }
 
-func NewInfo() Info {
+// Version returns the current version as a comparable version string.
+func (i Info) Version() VersionString {
+	return CurrentVersion.Version()
+}
+
+// Generator a Hugo meta generator HTML tag.
+func (i Info) Generator() template.HTML {
+	return template.HTML(fmt.Sprintf(`<meta name="generator" content="Hugo %s" />`, CurrentVersion.String()))
+}
+
+// NewInfo creates a new Hugo Info object.
+func NewInfo(environment string) Info {
+	if environment == "" {
+		environment = EnvironmentProduction
+	}
 	return Info{
-		Version:    CurrentVersion.Version(),
-		CommitHash: CommitHash,
-		BuildDate:  BuildDate,
-		Generator:  template.HTML(fmt.Sprintf(`<meta name="generator" content="Hugo %s" />`, CurrentVersion.String())),
+		CommitHash:  commitHash,
+		BuildDate:   buildDate,
+		Environment: environment,
 	}
 }

common/hugo/hugo_test.go

@@ -23,12 +23,13 @@ import (
 func TestHugoInfo(t *testing.T) {
 	assert := require.New(t)
 
-	hugoInfo := NewInfo()
+	hugoInfo := NewInfo("")
 
-	assert.Equal(CurrentVersion.Version(), hugoInfo.Version)
-	assert.IsType(VersionString(""), hugoInfo.Version)
-	assert.Equal(CommitHash, hugoInfo.CommitHash)
-	assert.Equal(BuildDate, hugoInfo.BuildDate)
-	assert.Contains(hugoInfo.Generator, fmt.Sprintf("Hugo %s", hugoInfo.Version))
+	assert.Equal(CurrentVersion.Version(), hugoInfo.Version())
+	assert.IsType(VersionString(""), hugoInfo.Version())
+	assert.Equal(commitHash, hugoInfo.CommitHash)
+	assert.Equal(buildDate, hugoInfo.BuildDate)
+	assert.Equal("production", hugoInfo.Environment)
+	assert.Contains(hugoInfo.Generator(), fmt.Sprintf("Hugo %s", hugoInfo.Version()))
 
 }

common/hugo/version.go

@@ -130,8 +130,8 @@ func BuildVersionString() string {
 	program := "Hugo Static Site Generator"
 
 	version := "v" + CurrentVersion.String()
-	if CommitHash != "" {
-		version += "-" + strings.ToUpper(CommitHash)
+	if commitHash != "" {
+		version += "-" + strings.ToUpper(commitHash)
 	}
 	if isExtended {
 		version += "/extended"
@@ -139,14 +139,12 @@ func BuildVersionString() string {
 
 	osArch := runtime.GOOS + "/" + runtime.GOARCH
 
-	var buildDate string
-	if BuildDate != "" {
-		buildDate = BuildDate
-	} else {
-		buildDate = "unknown"
+	date := buildDate
+	if date == "" {
+		date = "unknown"
 	}
 
-	return fmt.Sprintf("%s %s %s BuildDate: %s", program, version, osArch, buildDate)
+	return fmt.Sprintf("%s %s %s BuildDate: %s", program, version, osArch, date)
 
 }