_config.yml: delete default config & refactor
locale_filter plugin: move locale date from _config.yml to _data/locale
post_yaml_cache: update cache even if mtime changed & content not changed
delete multi-lang and generate one lang only for one execution

* _config.yml now defines base config.

* _config.ja.yml and _config.en.yml defines language config.
  To build Japanese pages, run:
  > jekyll build --config _config.yml,_config.ja.yml

* fix generators plugin.

* delete post_multilang_by_category plugin.
Gruntfile: build both ja & en pages
stop generating multi language pages

Now generate one language pages for one execution.

Generate Japanese page:
  > jekyll build --config _config.yml,_config.ja.yml

Generate English page:
  > jekyll build --config _config.yml,_config.en.yml