Change the agnoster theme for more configurability

The agnoster theme now supports:

* Setting color for all kinds of elements, by setting an appropiate config variable to the color name in your zshrc
  Example: AGNOSTER_GIT_DIRTY_BG=red # Set the git prompt background color to red for dirty repos
* Allowing to separate the git root and the relative path in your prompt by setting AGNOSTER_GIT_INLINE to 'true'
  You will then have a segment for the git root (ie. ~/.oh-my-zsh) followed by the existing git branch segment (ie. master) and
  finally the relative directory path (ie. themes)
* By setting AGNOSTER_STATUS_RETVAL_NUMERIC to 'true', The return value in the status prompt will become a number instead of a cross.

This will not affect people using this theme, as with default options, the theme behaves exactly as before this change.