Zsh support

At present, prm is mostly developed and tested with bash, as this is what I personally use and have the most experience with. I'm very much open to contributions that fix issues with zsh (provided that they don't break bash support), and I try to test prm using it every once in a while to the best of my abilities. As far as I know, prm currently works with zsh.

Tests are currently being implemented (see issue #11), in order to ensure full support.