Add plugin-pyenv as an available function for fish

[pantuza]

This pull request has the following contributions:

• Refactor the init script to become a function;
• Create documentation of how to install and use plugin;

[pantuza]

The branch rehab seems really complete for me. Is there any problem at why it is not in the master?

It would be a good version of an official plugin.

[scorphus]

All it lacks is a proper README.md and some test drive on a GNU/Linux box, which I could do later tonight.

[scorphus]

And thank you for testing it! Much appreciated 👍

[brunobbbs]

The branch rehab also works great for me. Thanks!

[scorphus]

Thanks for confirming it works, fellas!

Last night I had a chance to test it on a Linux box. It works there too only after a few tweaks. So rehab seems to work regardlessly of the install method – package manager or pyenv-installer.

But it still lacks a proper README.md. And I think we can gain from the one of this very PR. What do you think, @pantuza?

[pantuza]

@scorphus I think that would be awesome. If you need any help, let me know!

[scorphus]

Hey, dear fellas, @brunobbbs and @pantuza!

I've updated the rehab branch and created a pull request (#5). Can you please review it?

Thanks!