Add prepend option

I love this module. This is the only thing stopping it from being perfect, IMO.

This change allows you to inject styles that have a lower precedence than user styles, which is exactly what you'd want if you're creating a modular theming system, which I'm currently doing.

I've fully tested this change and documented it in the readme, trying my best to match your writing style. Let me know if you'd like me to make any tweaks.