Make ctrlp#call return the result

This is useful to make extensions/custom match functions to execute
internal methods like MatchIt

Closes #315
Add an option for saving the MRU list on every update

Closes #317
Allow x_func options to be defined using dict functions

Closes #319
Further reduce MRU saving

Only save once when a new entry is added.
Check and insert the ending slash

Fixes #321
Update ctrlp.txt
Set pos when data is empty

Fixes #323
Fix for recognizing/expanding of environment variables

Thanks to Simon Ruderich
Check bufnr and bufname before closing

Refs #335
Reset 'cscopetag' temporarily while using `:tag`

Refs #235
Run :tag as if typed

Refs #339
The message can be useful
Version 1.79