Mime type for HTML files and support index.html

I have read #93 and in general agree wholeheartedly with the Unix philosophy. Nevertheless, since it's just 2 lines, it would be great to also serve html files with the correct mime type, and handle index.html files.

Note that this means the browser has to be directed to, e.g., http://localhost:35729/foo/bar.html

At least for my personal use case that consists mostly of js and css development (without any ruby server technology), this makes it considerably easier to get started.