Added Rails Designer's attractive.js

[eelcoj]

This adds https://github.com/Rails-Designer/attractive.js as a JS library. It works like this:

<button data-action="toggleClass#active" data-target="#target">
  Toggle
</button>

<p id="target" class="target">
  This element should toggle the `active` class when the above button is clicked.
</p>

[adamghill]

If you update this PR and make it mergeable, I'll merge it in.

[eelcoj]

Done ✅