Auto-ignore certain editor files

[cespare]

Tv` asked for this for emacs on IRC. Looking at this emacs wiki page, it looks like I should ignore:

• \.#.*$
• ^#.*#$

We also might cover vim's backup and swap files:

• .*~$
• .*\.swp$

I suppose we would need a flag to un-ignore them, but maybe I won't bother unless someone complains.

[cespare]

Maybe also ignore .git (and below), and others? (.hg, etc)