Tweak gitignore

Summary: tags files should be ignored at all subdirectory depths; sometimes I want to
limit my search to a small subset of files, e.g. when working on Hack I only
want a tags file for hphp/hack/src

I am also using a .vimrc with project-specific configuration, so it would be
nice to have that gitignored too.

Reviewed By: @ptarjan

Differential Revision: D1557634