Magit support

[dcrewi]

Implementation credit goes to @binarin, but I'm requesting a merge because I find it useful. Admittedly, special-casing magit-status-mode buffers smells a little wrong, but the same thing is done with dired buffers. Ideally, there'd be some macro that provides a generic way to add special case buffers to the tests in eproject--buffer-file-name, but I haven't spent any time on it because this works for me.

[jrockway]

Interesting. I think it's probably better to come up with an extension mechanism around eproject-maybe-turn on, and then put the magit stuff in a separate file that uses that extension mechanism. Then we don't require magit (or confuse the byte compiler with warnings about the magit symbols not existing), and users can easily control the features. And other extensions will be able to do something similar without modifying eproject.el directly.