Provide something that would support URL fingerprinting

There is as yet nothing in place to support URL fingerprinting (see _[Use fingerprinting to dynamically enable caching](https://developers.google.com/speed/docs/best-practices/caching)_) of stuff like CSS files.

The point is to allow, e.g., a hash to be appended to `style.css` to give, say, `style.css.835042053c5ec545d6eb3946d8f8669aac9d87b5`. This could then be used to have long cache expiry times without having to worry about caches becoming stale.

It's possible that a custom filter (see #7) would allow this.