Extend PrtInsert to support multi character action

From https://github.com/kien/ctrlp.vim/pull/568:

>    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)