[RFC] Introduce CSS modules & covert /src/options over to using them

[aquibm]

Why?

This approach is much less restrictive than using the css-in-js approach that we had earlier. We're simply writing css here and can therefore easily style pseudo elements and make use of css animations. It also brings in new css features (variables, nesting, etc) and follows the golden rule that all styles are local to the component unless explicitly made global. (see routeTitle in base.css)

Changes:

• The cssOutput prop in the bundle determines whether we'll attempt to build css-modules.
• Stop tracking extension/options/style.css as this is now automatically generated via css-modulesify.
• Covert over all the css-in-js styles in options to use css-modules.
• Add autoprefixing and other features via cssnext

Future Tasks:

• Convert the overview app to use css-modules

RFC?

This is the first time I've used css-modules through browserify, not sure if it's setup correctly. @Treora, please let me know if I need to change anything 👍

[Treora]

I am starting to like the idea of css-modules, to organise style in the same hierarchy and to make it traceable where a component's style is defined. It looks like css-next gives the main features I wanted to use scss for, but now with at least the pretension that it will be normally supported css some day.

If you like this approach I am happy to apply it to the whole project. In any case this seems better than either inline styles defined in javascript or having a single css file.

[Treora]

Apart from these details, would this be ready to merge?

[Treora]

By the way, feel free to squash all the tiny fixes in a single commit, keeping up the appearance that everything was perfect at the first shot. ;)

[aquibm]

Yep, this is good to merge :shipit: