Extend PrtInsert to support multi character action

[d11wtq]

From kien#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)

[tacahiroy]

I don't imagine this is worth in which situation.
Any thoughts?

[Kris2k]

I use it on rare occasion mainly for include/import directive.
I think I made it out of habit that I am able to insert into cmdline directly.
I used this mainly for buffer management. But whole process was not scripted, and manually typed.
Something across the lines
yiw:buff "
when cursor was over include directive.
I did it for my own use, I put merge request to see how others feel/need such a thing.