Extend PrtInsert to support multi character action

    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)