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..

1 files changed, 48 insertions, 0 deletions

diff --git a/hugolib/multilingual.go b/hugolib/multilingual.go
new file mode 100644
index 000000000..2552bad4c
--- /dev/null
+++ b/hugolib/multilingual.go
@@ -0,0 +1,48 @@
+package hugolib
+
+import (
+	"github.com/spf13/cast"
+	"github.com/spf13/viper"
+)
+
+type Multilingual struct {
+	enabled bool
+	config  *viper.Viper
+
+	Languages []string
+}
+
+func (ml *Multilingual) GetString(key string) string { return cast.ToString(ml.Get(key)) }
+func (ml *Multilingual) GetStringMap(key string) map[string]interface{} {
+	return cast.ToStringMap(ml.Get(key))
+}
+
+func (ml *Multilingual) GetStringMapString(key string) map[string]string {
+	return cast.ToStringMapString(ml.Get(key))
+}
+
+func (ml *Multilingual) Get(key string) interface{} {
+	if ml != nil && ml.config != nil && ml.config.IsSet(key) {
+		return ml.config.Get(key)
+	}
+	return viper.Get(key)
+}
+
+func (s *Site) SetMultilingualConfig(currentLang string, orderedLanguages []string, langConfigs map[string]interface{}) {
+	conf := viper.New()
+	for k, val := range cast.ToStringMap(langConfigs[currentLang]) {
+		conf.Set(k, val)
+	}
+	conf.Set("CurrentLanguage", currentLang)
+	ml := &Multilingual{
+		enabled:   len(langConfigs) > 0,
+		config:    conf,
+		Languages: orderedLanguages,
+	}
+	viper.Set("Multilingual", ml.enabled)
+	s.Multilingual = ml
+}
+
+func (s *Site) multilingualEnabled() bool {
+	return s.Multilingual != nil && s.Multilingual.enabled
+}