Allow overriding the renderer when instantiating Powerline

This is useful to allow e.g. the prompt script to use a common shell
theme while having different renderers for different shells.
Rename "terminal" extension to "shell"

Refs #84.
Add zsh prompt renderer

Zsh can use regular escape sequences in the prompt, but they must be
wrapped in %{...%} to work correctly.

Refs #84.
Allow side, extension and renderer module to be specified in powerline-prompt

Refs #84.
Update zsh prompt example in docs