Upgrade dependencies and support the webpack 4 plugin system. Didn't make any attempt to support earlier versions of webpack (maybe should add webpack ^4.0.0 as a peer dependency?).
Needed to tweak the tests to get them to pass, and needed to update the jest configuration slightly.
Hey @rkilgore-meta , I rebased your PR to cut out the work on excluding directories but couldn't push to your remote I think so I had to push to a new branch here and open a new PR 😅
fixes #29