Add cache feature. the idea from #27

Note: This implementation is rough, so refactoring required
Fix MRU feature doesn't work problem
Change compare method to timestamp and file size

This way is suitable for older Vim
This should be a script local func
Add Windows support for cache feature
Care about Windows. #27

pointed out by @dusans
Better naming?
Non files should be excluded for caching. #27
Add doc for cache feature (#27)
Merge branch 'add-cache-feature'

Conflicts:
	doc/ctrlp-funky.txt