Add support for Zsh

[Jefferson-Faseler]

Branch off of #10
Couldn't target branch in @jordanwilson230's repo and didn't want to make @grzesuav the middleman.

Checklist

1. Addresses Issue: Make kubectl-prompt suitable for use with zsh #9
2. Instead of dependencies on the ~/.bash_profile throughout the code, the scripts can dynamically interpolate the shell profile. As of now support is Zsh and bash as the default.
3. Changes are GNU/BSD Compatible
4. Changes are applicable to the wider community.: I believe this is covered by the feature request.
5. Request help with testing if needed.
   I tested against prompt and it's family of sub-commands and flags, as well as image.

[jordanwilson230]

@Jefferson-Faseler @grzesuav thanks for contributing! I see you were able to test against the prompt and image plugins but wanted to ask if anyone would be able to test the ssh plugin as well?

[jordanwilson230]

@Jefferson-Faseler @grzesuav What do you think about just moving the zsh stuff into its own branch and putting linking it in the readme. Would you be ok if I put your changes in a zsh branch?

[Jefferson-Faseler]

Let me do a bit more testing on it. Had some odd warnings from when I jumped to a bash env earlier today from zsh and want to make sure it wasn't from the way I'm doing something here.