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

commands/commands.go

@@ -14,6 +14,11 @@
 package commands
 
 import (
+	"os"
+
+	"github.com/gohugoio/hugo/hugolib/paths"
+
+	"github.com/gohugoio/hugo/common/hugo"
 	"github.com/gohugoio/hugo/common/loggers"
 	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/helpers"
@@ -159,6 +164,7 @@ Complete documentation is available at http://gohugo.io/.`,
 	})
 
 	cc.cmd.PersistentFlags().StringVar(&cc.cfgFile, "config", "", "config file (default is path/config.yaml|json|toml)")
+	cc.cmd.PersistentFlags().StringVar(&cc.cfgDir, "configDir", "config", "config dir")
 	cc.cmd.PersistentFlags().BoolVar(&cc.quiet, "quiet", false, "build in quiet mode")
 
 	// Set bash-completion
@@ -185,8 +191,9 @@ Complete documentation is available at http://gohugo.io/.`,
 }
 
 type hugoBuilderCommon struct {
-	source  string
-	baseURL string
+	source      string
+	baseURL     string
+	environment string
 
 	buildWatch bool
 
@@ -200,15 +207,45 @@ type hugoBuilderCommon struct {
 	quiet      bool
 
 	cfgFile string
+	cfgDir  string
 	logFile string
 }
 
+func (cc *hugoBuilderCommon) getConfigDir(baseDir string) string {
+	if cc.cfgDir != "" {
+		return paths.AbsPathify(baseDir, cc.cfgDir)
+	}
+
+	if v, found := os.LookupEnv("HUGO_CONFIGDIR"); found {
+		return paths.AbsPathify(baseDir, v)
+	}
+
+	return paths.AbsPathify(baseDir, "config")
+}
+
+func (cc *hugoBuilderCommon) getEnvironment(isServer bool) string {
+	if cc.environment != "" {
+		return cc.environment
+	}
+
+	if v, found := os.LookupEnv("HUGO_ENVIRONMENT"); found {
+		return v
+	}
+
+	if isServer {
+		return hugo.EnvironmentDevelopment
+	}
+
+	return hugo.EnvironmentProduction
+}
+
 func (cc *hugoBuilderCommon) handleFlags(cmd *cobra.Command) {
 	cmd.Flags().Bool("cleanDestinationDir", false, "remove files from destination not found in static directories")
 	cmd.Flags().BoolP("buildDrafts", "D", false, "include content marked as draft")
 	cmd.Flags().BoolP("buildFuture", "F", false, "include content with publishdate in the future")
 	cmd.Flags().BoolP("buildExpired", "E", false, "include expired content")
 	cmd.Flags().StringVarP(&cc.source, "source", "s", "", "filesystem path to read files relative from")
+	cmd.Flags().StringVarP(&cc.environment, "environment", "e", "", "build environment")
 	cmd.Flags().StringP("contentDir", "c", "", "filesystem path to content directory")
 	cmd.Flags().StringP("layoutDir", "l", "", "filesystem path to layout directory")
 	cmd.Flags().StringP("cacheDir", "", "", "filesystem path to cache directory. Defaults: $TMPDIR/hugo_cache/")