Extend PrtInsert to support multi character action

[Kris2k]

PrtInsert can be mapped to insert clipboard or some register.
This extends PrtInsert to support insert any register (e.g "b
register).
Rationale: I mostly use " (unnamed register for work) so now
let g:ctrlp_prompt_mappings = {
    \ 'PrtInsert("r\"")': ['<c-u>'],
    \ }
will work.
( when user enters c-u the content of unnamed register is entered)

[d11wtq]

Moved to ctrlpvim#13