Add a config option to specify a base directory for the stylesheets

[Blond11516]

Based on the examples in the readme, you could set it to "assets/css". This reduces the boilerplate for using the library a little bit as well as making it easier to move your whole assets management around, should you need to.

The current implementation seems to favor colocated stylesheets rather than grouping them under something like assets/css. I think it's nice to have that option, so I'd like for both to be supported. What I'm imagining is a stylesheet_base_path config option that could be set to a String or to :same_as_view (which would be the default for backwards compatibility).