[FR] Simple clean url support

[Ssamm45]

Some static site generators output expecting the server to have pretty URLs turned on. Meaning a request to /about would serve /about.html.

It could be useful to add an option to, when given a URL with no extension, check if the file exists with the given extension. darkhttpd --default-extension 'html'.

[emikulic]

I like to just make it a directory so the URL looks like /about/ :)

If you have a strong need for this, I'd be willing to accept a patch. Please also write a test.