[prompt] steef: Update it to be customizable
[prompt] steef: Update it to be customizable

and to have a help and a preview too.

Inspired by #192, and by having (few but) good prompt themes to serve as
starting points for the users to customize or implement their own.

Also removed the colors functions, since we don't need that. We're using
the newer '%F{red}color%f' code instead. (Actually all our themes are)

Closes #245

Eriner/zim,joshuarubin/dotfiles,joshuarubin/dotfiles,joshuarubin/dotfiles,joshuarubin/dotfiles