Better completion engine (YouCompleteMe plugin)

[xentac]

It would be really neat if we could use all the benefits of the YouCompleteMe background processing and autofilling as well as fast matching. I'd like to write this as a compliment to the existing completion methods.

My plan is to support both cases--default omnifunc and YouCompleteMe Completer API--the latter being able to refresh the account list in the background and matching as you type.

[yagebu]

@xentac I've implemented this for deoplete (which I prefer to YCM) in my PR.