Add tests

[Mr0grog]

This tool needs tests! Making changes has always been fiddly, and there should really be some automated tests to make sure things don’t break. Since folks are also looking for library-style and CLI usage in the future (see #42), they need to be able to run both in browsers and in Node.js.

• Unit tests on the actual conversion of some Google Docs HTML strings → Markdown strings are the most important thing here.
• Some real simple end-to-end tests (“I pasted something in the paste box and got the right markdown in the output box”) would be great, too, but aren’t critical.
• We need some fixtures that are actual Google Docs. The HTML formats we get from copying or exporting might change over time, so we really want an actual doc to test against. We might add a script to download the docs as HTML to store in the repo, so we can easily update the fixtures any time.
• Technologically, Karma + Mocha is probably the most straightforward here, since the tests can be written in a way that aren’t tied to a browser (because we also eventually want CLI usage, which means running in Node.js, too).