Comment file generation for MovableType

- comments option added (currently default false)
- metadata for entry added: blog_id, post_id, mt_id, basename
  note: post_id is same as mt_id. for the comment plugin
- comments formats are same as posts: with YAML front matter
- comment related metadata/content output methods and path generators
- also fixes default option processing order
Option value hash keys should be string
Description of the options for MovableType importer
Merge branch sigeya:mt_comment_hack into jekyll:shigeya-mt_comment_hack

Merges #66. Closes #66