Update to follow sample plugin closer

[jclusso]

I was updating this plugin to get it working with current versions of Bridgetown and realized it differs quite a bit from the sample plugin. I managed to get everying sorted out in #41, but it was a bit more difficult since this project differs quite a lot. What are the thoughts on updating this plugin to match the structure, testing, etc of the sample plugin. I have quite a bit of the changes already locally from working on #41, but decided it wasn't appropriate to make those sorts of changes without suggesting it first.