make ShowIndex work without libxslt

Hi Hugo,

When I installed Hiawatha for the first time, I thought it would make dir listing as its builtin functionality, but it turned out I have to enable libxslt support for that. Compare to nginx and lighttpd, it seems to me that's a little bit complex for that simple task. So here I submitting my pull request to let Hiawatha list directories without XSLT support.

However, that could be just me...and, what do you think about that idea?
Let me know if there's anything further I could do. Thanks.

H.