Source maps support

[Dremora]

I guess this depends on css-modules/css-modules-loader-core#59.

[dfreeman]

Enabling source maps also has some potential perf implications, but I'd like to at least have the option to turn them on as well. Once css-modules/css-modules-loader-core#59 lands we can investigate and see what's feasible without hurting build times too badly.