Use webpack

[saiichihashimoto]

• Build entire extension (css, js, json, all of it)

Instead of using browserify, uglify, watchify, etc. convert the extension to using webpack.

Currently, there's too many separate parts doing their own thing (i.e. watchify, postcss, copying some files). That's what webpack is made for and it makes development for it great. Not to mention, building for deployment is much simpler.

Blocked by #10.