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

htesting/test_structs.go

@@ -39,7 +39,7 @@ func (t testSite) Language() *langs.Language {
 // NewTestHugoSite creates a new minimal test site.
 func NewTestHugoSite() hugo.Site {
 	return testSite{
-		h: hugo.NewInfo(),
+		h: hugo.NewInfo(hugo.EnvironmentProduction),
 		l: langs.NewLanguage("en", newTestConfig()),
 	}
 }

htesting/testdata_builder.go

@@ -0,0 +1,59 @@
+// 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 htesting
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/spf13/afero"
+)
+
+type testFile struct {
+	name    string
+	content string
+}
+
+type testdataBuilder struct {
+	t          testing.TB
+	fs         afero.Fs
+	workingDir string
+
+	files []testFile
+}
+
+func NewTestdataBuilder(fs afero.Fs, workingDir string, t testing.TB) *testdataBuilder {
+	workingDir = filepath.Clean(workingDir)
+	return &testdataBuilder{fs: fs, workingDir: workingDir, t: t}
+}
+
+func (b *testdataBuilder) Add(filename, content string) *testdataBuilder {
+	b.files = append(b.files, testFile{name: filename, content: content})
+	return b
+}
+
+func (b *testdataBuilder) Build() *testdataBuilder {
+	for _, f := range b.files {
+		if err := afero.WriteFile(b.fs, filepath.Join(b.workingDir, f.name), []byte(f.content), 0666); err != nil {
+			b.t.Fatalf("failed to add %q: %s", f.name, err)
+		}
+	}
+	return b
+}
+
+func (b testdataBuilder) WithWorkingDir(dir string) *testdataBuilder {
+	b.workingDir = filepath.Clean(dir)
+	b.files = make([]testFile, 0)
+	return &b
+}