_config.yml: change pygments to highlighter
hatena converter: update to Jekyll 2.2 (highlighter in _config.yml)
post_to_liquid_raw: Hash#deep_merge -> Jekyll::Utils.deep_merge_hashes
amazon tag: Update to Jekyll 2.2

In Jekyll 2.2, REXML::Document is not required by default
plugins: disable temporarily
site_ext: quick hack for HTML backward compatibility
site_ext: delete Post#source & use Post#path instead
post_multilang_by_category: fix Post#next & Post#previous order

On Jekyll 2.2, values of the hash Post#categories are sorted decending
order and created when we refer to it.
archives plugin: change to a Generator
tags plugin: change to a Generator
lang plugin: change to a Generator
Move generator plugins from _plugins dir to _plugins/generators dir
Fix timezone: UTC -> JST (+0900)
Remove unused static file help.html (generated by help.md)
Remove ordered list
Fix markdown format to work on redcarpet v3.1.2
RedCarpet 3.1.2 で正しく表示できるように Markdown を整形した
Modify HTML to avoid RedCarpet convert bug
Fix see also URL
「あわせてよみたい」のリンク先が英語版になることがあったのを修正した