zshrc: support prompt shortening / shrink-path

[mika]

Forwarding this from a personal mail I received:

Could you make it possible to shorten the prompt paths in grml zsh?
For inspiration, please look at a plugin from oh-my-zsh:
https://github.com/robbyrussell/oh-my-zsh/tree/master/plugins/shrink-path

We cut the path at 40 characters already, if that would be more configurable and possibly also support a zstyle setting to use something like in the shrink-path plugin (without having to use oh-my-zsh :)) that might be indeed nice.

If someone knows how to properly do that, I'd be more than happy to accept patches. :)