Minor improvements

Hey Karl, having switched to this plugin now, here are two minor improvements.

I've added a switch to disable the recursive bib search. This significantly speeds up \cite-completion on large documents, and I would imagine that this is a common setup with one main file linking to all bib files.

Also, I added a suffixesadd to tex.vim to allow navigating between include/input files using `gf`. I think in LaTeX-Box this was achieved using some other method, but this seems the simplest solution to that.