Installs Node.js and ends the setup process automatically

Many people around me don't read enough of the README and expect Node.js to be installed after running the install script.

Based on a Gist I've written for my coworkers (https://gist.github.com/xcambar/2f88b912b7d40fe605c5), we thought it could be beneficial to NVM that the install script actually installs Node.js.

This is what this PR does, with a configurable Node.js version (defaulting to `stable`).
:+1: too, but as it was the former behaviour of the install script, it felt out of scope to remove it without notice.

As we seem to agree on it, I'll add the `source`.