allow users to inject custom CSS

[sunaku]

In lieu of issues #205, #206, and #207, please add a setting that allows the user to inject their own custom CSS (which would override the current theme and css/annotations.css styles) into the book.

[rkwright]

We'll look into this.

[sunaku]

Thanks! For the time being, users can work around this limitation by cloning this git repository, modifying the css/* files, and building (and using/installing) the Chrome extension locally. 🍰