Run test suite in node

[yagni]

Even though the library has node support baked in, all of the tests run in Karma, which is browser-only. We need to also run the test suite for Node.js to make sure everything is still working. We're already using Mocha so it may be easiest to use for node tests. This is a prerequisite to updating Node.js and finding Node-related deprecations.

instant-mocha looks promising, but will require a webpack update.

Reference: #85