CSS support

Right now there is no way to add custom CSS. At least not a great way. I see there being two paths on how to handle CSS.

* traditional stylesheets that are compiled into a single stylesheet - this is the most complicated method but would probably be the most accepted. Basically there would need to be a way to take each style sheet represented as an AST that are merged into a single AST then output as a single stylesheet that can be accessed via a URL. Implementation-wise this could be compiled functions similar to the Heex compilation method we're using. We'd need an CSS parser though, preferably without a C compilation requirement.
* inline styles - this one will likely get some hate and I don't disagree. The inline styles would definitely increase the bytes per page. This method would certainly be easy to implement and far cleaner.