Add multilingual support in Hugo

Implements:
* support to render:
  * content/post/whatever.en.md to /en/2015/12/22/whatever/index.html
  * content/post/whatever.fr.md to /fr/2015/12/22/whatever/index.html
* gets enabled when `Multilingual:` is specified in config.
* support having language switchers in templates, that know
  where the translated page is (with .Page.Translations)
  (when you're on /en/about/, you can have a "Francais" link pointing to
   /fr/a-propos/)
  * all translations are in the `.Page.Translations` map, including the current one.
* easily tweak themes to support Multilingual mode
* renders in a single swift, no need for two config files.

Adds a couple of variables useful for multilingual sites

Adds documentation (content/multilingual.md)

Added language prefixing for all URL generation/permalinking see in the
code base.

Implements i18n. Leverages the great github.com/nicksnyder/go-i18n lib.. thanks Nick.
* Adds "i18n" and "T" template functions..

tpl/template_funcs.go

@@ -1920,5 +1920,7 @@ func init() {
 		"upper":        func(a string) string { return strings.ToUpper(a) },
 		"urlize":       helpers.URLize,
 		"where":        where,
+		"i18n":         I18nTranslate,
+		"T":            I18nTranslate,
 	}
 }

tpl/template_i18n.go

@@ -0,0 +1,47 @@
+// Copyright 2015 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 tpl
+
+import (
+	"fmt"
+
+	"github.com/nicksnyder/go-i18n/i18n/bundle"
+	jww "github.com/spf13/jwalterweatherman"
+)
+
+var i18nTfunc bundle.TranslateFunc
+
+func SetI18nTfunc(lang string, bndl *bundle.Bundle) {
+	tFunc, err := bndl.Tfunc(lang)
+	if err == nil {
+		i18nTfunc = tFunc
+		return
+	}
+
+	jww.WARN.Printf("could not load translations for language %q (%s), will not translate!\n", lang, err.Error())
+	i18nTfunc = bundle.TranslateFunc(func(id string, args ...interface{}) string {
+		// TODO: depending on the site mode, we might want to fall back on the default
+		// language's translation.
+		// TODO: eventually, we could add --i18n-warnings and print something when
+		// such things happen.
+		return fmt.Sprintf("[i18n: %s]", id)
+	})
+}
+
+func I18nTranslate(id string, args ...interface{}) (string, error) {
+	if i18nTfunc == nil {
+		return "", fmt.Errorf("i18n not initialized, have you configured everything properly?")
+	}
+	return i18nTfunc(id, args...), nil
+}