Added support for Zsh

[brianmatzelle]

Hi! I found this to be useful, so I converted the bash script to zsh and added supporting tests + documentation for it.

I duplicated the bash documentation into zsh sections, rather than merging instructions. I figured it would be best for you to decide what direction to take the documentation in, so I tried not to only add -- not edit/remove anything.

If you merge the fork, make sure to change the links called by the curl request right after merging. They're currently pointing to my fork, but should point to the main repo afterwards.

[brianmatzelle]

One question, was it intentional to have the shell exit into nothingness after leaving the directory with .local.bashrc in it?

I'd prefer if it returned back to the parent shell after exiting, so I altered the script on my local machine to do that. I didn't merge it because I'm not sure if you intentionally detached the .local shell. I don't want to disrespect/alter any design choices.