Allow for suffix-less file paths to return html content

@magnars assuming this is not something you would want given the explicit design, but thought I might as well confirm.

If it is something you'd be interested in I could clean this up (`statically-servable-uri?`, `project.clj`, etc) or could simply better explain what I'm trying to achieve. 

I'm pretty new to Clojure, if that isn't already apparent, though, so *buyer beware*.