[refactor, chore] "Compile" CSS via Webpack

This may seem unnecessary (and odd to import CSS files into JS...), but
this sets up the project for being able to keep the distributed
(release) resources all together, as well as setting the stage for
introducing something like SCSS.

package.json

@@ -51,6 +51,7 @@
     "less": "^3.0.4",
     "less-loader": "^4.1.0",
     "lodash": "^4.17.10",
+    "mini-css-extract-plugin": "^0.4.1",
     "mkdirp": "^0.5.1",
     "mocha": "^3.2.0",
     "mocha-lcov-reporter": "^1.0.0",