Support CMB batteries

Thanks to @tempookian for his pull request.

Merged with stablestud

This versions incorporate several additions/fixes by @stablestud, @RodgerOliver, and @bast, including:

• setting the font for the i3 nagbar
• setting notification interval
• more general shebang (support nixos)

First release

Merge pull request #17 from rthiago/master

Fix bug with notify-send thanks to rthiago