Modular builds

Added `-c/--css` and `-j/--js`

If any of these flags are enabled, the corresponding file will be built. If none
are enabled, everything will be built.

Useful for build systems that only rebuild the things that need rebuilding, like
djb-redo. It's doubly useful the component output is a dependency to a larger build.

Tiny bit of code smell here, the component-builder abstraction had to leak into component-build a tiny bit to express what I was trying to do.