Add homebrew support

[bylatt]

It's will be great if I can install this via homebrew.

[jasonkarns]

I'm one of the maintainers of another node version manager that is also a direct fork of rbenv (https://github.com/OiNutter/nodenv). It is under active maintenance and stays current with changes in rbenv upstream. And is available in homebrew as nodenv. https://github.com/Homebrew/homebrew/blob/master/Library/Formula/nodenv.rb