Add IPython prompt support
Add output_raw argument to Renderer.render() method

Update all ipython modules to use it to get unhighlighted prompt

Conflicts:
	powerline/renderers/ipython.py
Remove placeholder and use escaping instead
Fix zsh prompt security and other issues

* Escape percent in zsh prompt
* Prevent some security issues (directory named `$(echo abc)` appeared as `abc`)
    (`"$()"` → `'$()'`)
* Removed exports
Remove "$()" method in pre-prompt hook

Using "$()" method on pre-prompt hook is just asking for security
trouble.
Restore export as it makes sense in bash

Bash has weird initialization: bashrc is sourced in non-login
interactive shell, but not in login interactive shell; profile is
sourced in login interactive shell, but not in non-login interactive,
thus exporting in profile does make sense. In zsh case zshrc is always
sourced when shell is interactive and exporting thus makes no sense.
Add setopt promptpercent and promptsubst

These options are required for Powerline to work in zsh, but the latter
is off by default.
Add last_status and last_pipe_status segments
Make highlight_group optional if set in segment function