Add better tmux examples

The current tmux example script/configuration only allows Powerline to configure the left side. These changes make the example `pl.py` script able to output segments for both sides, as well as making the script slightly more extensible. I've used this script to get Powerline looking much like [tmux-powerline](https://github.com/erikw/tmux-powerline) (with a few custom segments).

![enhanced tmux config](https://f.cloud.github.com/assets/77758/57901/1cb245d6-5b5d-11e2-8ac1-1975b41e6918.png)