CSS support

[mstade]

We need a build step for css, for a few reasons:

• We'd like to enable future css features, where it is sensible
• We need to generate a concatenated css file for production builds, in much the same way we have a concatenated js file
• Possibly enable additional tooling around css, such as css modules

[Poetro]

At least concatenate every file aggregated in the optimize step

[mstade]

Closing because #13.