feat: External source maps

[tivac]

Adding support for external source maps, trigger it by passing this option:

{
    map: {
        inline: false
    }
}

It'll automatically write to options.css + .map, not sure if I want to support customizing the output path/name atm.

WARNING: Webpack implementation is a giant giant hack.

[tivac]

This branch is causing travis builds to hang & I have zero clue why 😞