Add shell module

Add new "shell" module which can execute arbitrary shell command and display
its output.
Add charging_color and full_color parameters to the battery module
Add {bar} to the battery module

Add {bar} (from util.make_bar()) formatter to the battery module. Also removed
the final .strip() call as it was eating the padding used by make_bar.
Added {quality_bar} to wireless module
Added not_present_text and not_present_color parameters to the battery module

This commit also fixes i3pystatus crashing if the battery is removed while
running.
Merge pull request #76 from atalax/master

Add shell module