Add eslint plugin demo page

I sometimes want to write some arbitrary code and see what the Airbnb eslint configuration has to say about it. It would be nice to have a quick REPL that I could load up in the browser to run quick checks like this. Similar to https://babeljs.io/repl/

Start by reviewing the requested browser REPL and comparing its intended interaction with the linked Babel REPL example. Done means users can load a page, enter arbitrary JavaScript, and see diagnostics from the Airbnb ESLint configuration.

Written by the indexing model from the issue text.