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)

autoload/ctrlp.vim

@@ -681,6 +681,10 @@ fu! s:PrtInsert(...)
 		let regcont = s:getregs()
 		if regcont < 0 | retu | en
 	en
+	if type =~# '^r.$'
+		let regcont = s:regisfilter(type[1])
+		let type = 'r'
+	en
 	unl! s:hstgot
 	let s:act_add = 1
 	let s:prompt[0] .= type ==# 'w' ? s:crword