Add Pathogen/Vundle support

Closes #79.
Remove unnecessary import
Update usage docs

Refs #79.
Use code-blocks in docs
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
Make uptime tmux segment support psutil

psutil is used if available, and the function falls back to reading
/proc/uptime if not. According to the psutil docs this should work
across more platforms.
Make system load thresholds customizable
Fix zsh prompt example
Convert file name to UTF-8 in vim file name segment

Closes #87.
Merge branch 'develop' into local-develop