Feature request: Greg Hurrell's Zsh Prompt

[oblitum]

@wincent describes his great prompt setup here (link with highlighted comment). It would be nice if we got it wrapped up and integrated into Prezto.

I'm creating this issue just as a feature request as well as a note to self in case I find time to look how to wrap it up and create a pull request. If anyone is interested in it now though, feel free to implement it first.

Migrated from sorin-ionescu/prezto#1601