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

config/configLoader.go

@@ -0,0 +1,106 @@
+// Copyright 2018 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 config
+
+import (
+	"github.com/gohugoio/hugo/common/maps"
+	"github.com/gohugoio/hugo/parser/metadecoders"
+	"github.com/spf13/afero"
+	"github.com/spf13/viper"
+)
+
+// FromConfigString creates a config from the given YAML, JSON or TOML config. This is useful in tests.
+func FromConfigString(config, configType string) (Provider, error) {
+	v := newViper()
+	m, err := readConfig(metadecoders.FormatFromString(configType), []byte(config))
+	if err != nil {
+		return nil, err
+	}
+
+	v.MergeConfigMap(m)
+
+	return v, nil
+}
+
+// FromFile loads the configuration from the given filename.
+func FromFile(fs afero.Fs, filename string) (Provider, error) {
+	m, err := loadConfigFromFile(fs, filename)
+	if err != nil {
+		return nil, err
+	}
+
+	v := newViper()
+
+	err = v.MergeConfigMap(m)
+	if err != nil {
+		return nil, err
+	}
+
+	return v, nil
+}
+
+// FromFileToMap is the same as FromFile, but it returns the config values
+// as a simple map.
+func FromFileToMap(fs afero.Fs, filename string) (map[string]interface{}, error) {
+	return loadConfigFromFile(fs, filename)
+}
+
+func readConfig(format metadecoders.Format, data []byte) (map[string]interface{}, error) {
+	m, err := metadecoders.UnmarshalToMap(data, format)
+	if err != nil {
+		return nil, err
+	}
+
+	RenameKeys(m)
+
+	return m, nil
+
+}
+
+func loadConfigFromFile(fs afero.Fs, filename string) (map[string]interface{}, error) {
+	m, err := metadecoders.UnmarshalFileToMap(fs, filename)
+	if err != nil {
+		return nil, err
+	}
+	RenameKeys(m)
+	return m, nil
+}
+
+var keyAliases maps.KeyRenamer
+
+func init() {
+	var err error
+	keyAliases, err = maps.NewKeyRenamer(
+		// Before 0.53 we used singular for "menu".
+		"{menu,languages/*/menu}", "menus",
+	)
+
+	if err != nil {
+		panic(err)
+	}
+}
+
+// RenameKeys renames config keys in m recursively according to a global Hugo
+// alias definition.
+func RenameKeys(m map[string]interface{}) {
+	keyAliases.Rename(m)
+}
+
+func newViper() *viper.Viper {
+	v := viper.New()
+	v.AutomaticEnv()
+	v.SetEnvPrefix("hugo")
+
+	return v
+}