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`).
Only on "use" or "deactivate" - but what might be helpful here is to actually *source* `nvm`, rather than relying on the profile to do it?