Add subcommand to link/unlink selected version #83

Normally, I use the version of Node installed by Homebrew, and use Nodebrew to quickly switch between older Node.js versions when testing certain projects. When I'm finished testing something, I have to uninstall the current version if I want it out of my $PATH.

It'd be great if Nodebrew supported a command like brew unlink/brew link:

$ nodebrew unlink
Unlinked: ~/.nodebrew/current
$ nodebrew link
Linked: ~/.nodebrew/current -> ~/.nodebrew/node/v18.7.0

Alternatively, something like nodebrew use none could be added instead, which is arguably much more simple.