Protecting _filetype_completers with Lock

Multiple threads can try to create a single Completer at the same time.
That's bad. Like, really bad. It ended up not biting us in the ass
because of the way the code is structured (two different Completers for
the same filetype still work), but that was by accident.
Regex triggers now supported.

They are specified with a `re!` prefix. So `re!\w+` would use the `\w+`
regex.

Fixes Valloric/YouCompleteMe#1097