Add support for XBM icons.

[mad-s]

[LemonBoy]

Please have a look at the xbm [1] branch, with some nice tricks I've managed to avoid using xcb-image entirely. This said, I'll think about including this into the master but I'm more biased towards a no at the moment. There are some nice fonts that package a slew of symbols, let's put xbm to rest, it's a format that is meant to be parsed by a C compiler, not from a GUI program.

[1] https://github.com/LemonBoy/bar/tree/xbm