Added the Agnoster prompt as an option. 

See more info about the agnoster prompt here: https://gist.github.com/agnoster/3712874
It's not set as the default - but I would recommend it. It matches the theme of the status bar (Airline) in Vim 

There's probably a better place to add the parse_git_dirty() function - I just placed in zsh/git.zsh

Screenshot:
![Screenshot](https://gist.githubusercontent.com/agnoster/3712874/raw/5d28e2d9fe2e4d0a4fda0315ad97bdafa399425c/screenshot.png)