- Major change to file_mru

1. Time to save:
- Before: Save to file on every event
- Now: Use VimLeavePre to save.

2. maintain two list, short one for quick start; long one for keep history.
use `file_mru:*` or `all`, `long`, `_`  to load.

3.
Modularize and Merge file_mru / directory_mru into one
Rename file_mru directory_mru to mru/file and mru/directory
Merge branch 'feature/lazy_mru'

Conflicts:
	autoload/unite/sources/file_mru.vim
	plugin/unite/mru.vim