Investigating git status colouring.

- Learning bash scripting, to parse the output from git status.
- Some investigation of what that output actually is.
- Add the investigation results to the README.
Display the branch name in red if there are any changes.
Prompt now displays the index and working stats. Might be overkill, but it's the same as posh-git
Branch is now displayed in green if we're ahead of the remote.

Also, don't display the stats if they're zero.
Branch is now displayed in red if we're behind the remote, or yellow if we're both.
Display unknown files differently; add scratch3.
Display unknown files differently.