Add plugin for using nodenv

Hello again!

This plugin adds support for using nodenv (rbenv for nodejs). I realize that there is already the ndenv plugin, but nodenv is *slightly* different (it has a whole new vowel in the name! wow!) and made more sense for what I needed.

I hope that you will appreciate the prospect of having ***even more*** choice when it comes to the exciting world of node version management fish plugins!

Thanks so much.