Pythonification

As discussed, a rewrite of the go script in Python.

I took the liberty of adding colored output :)  Please check whether you find this Python code easy enough to read and understand, and whether the script works as intended.

Don't merge yet, let's first discuss, whether we also want to rewrite `bin/cask`.  It's not that long, but it has some tricky shell wizardry inside, and some complex nested quoting.  I think, it would be easier to handle in Python.

If you agree, I'd do this tomorrow, and update this PR.