Add wrapper function for tree to zshrc

This colorizes and auto-paginates (only when needed) the output of `tree`.

I mentioned this to @nurugger07 at rubyjax, and offered to submit a pull request for your consideration. I have not actually tested this exact configuration on a live machine (as I don't want to install dotmatrix), but the configuration is pulled straight from my zshrc, so it *should* work.