Add linux support

There's good news and there's bad news.

The good news is that adding linux support only took a dozen lines of code.
The bad news is that the project is named tmux-battery-osx instead of tmux-battery. You may want to change that if you accept this request.

All this does is use upower (which was installed on my machine and was the first result on google when I searched for how to check battery level from terminal on linux) instead of pmset.

I've also changed the default icons to things that aren't displayed as gibberish with the default font I have. You can decide whether or not you like them.