Split Powerline.__init__ into __init__ and create_renderer

Target: with long-living Powerline objects periodically reload configuration 
recreating renderer. Use file watchers to watch for configuration. Configuration 
should be able to be safely reloaded in non-blocking mode in a separate thread 
up to the time when it comes to recreating renderer.

This commit does not add anything that actually reloads the configuration, 
multiple runs of .create_renderer were not tested.
Take file_watcher from @kovidgoyal develop branch
Some fixes for flake8, remove executable bit and shebang
Automatically reload configuration

Needs testing