Working label completion with include

Should fix issue #73, at least it does so for me. If label completion with `\include`/`\input` is working for everybody else, I'll just keep this in my tree.

It also addresses an issue I've had where nonexistant .aux files would throw an error message: Now they're just ignored. I find this to be more useful for "big" projects which don't include everything at once and thus don't always have .aux files for all their inputs. I could probably add an option to switch between ignoring and throwing a warning message.