Integrate with sass

[kaj]

Make it possible to add the css generated from a sass file directly as a static file, and to referense other static files from that sass file (and get the correct hashed named in the generated css).

The aim of this PR is to have something like statics.add_sass_file("foo.sass") that automatically sassifies files and properly adds them.

let mut statics = StaticFiles::new(&out_dir).unwrap();
statics.add_files(&base_dir.join("static")).unwrap();
statics.add_sass_file("style.scss".as_ref()).unwrap();

The feature defines a sass function static-name that can be called with a source name of a static file to get the name including hash:

body {
    background: static_name(some.jpg);
}