Use file URLs for local docs

[mdorman]

Addresses #184, at least in my testing. It's also pretty darn obvious. :)

The question I could reasonably see it being worth asking is whether we should check and make sure that, say, index.html exists in the directory.

[ndmitchell]

Thanks - that does seem way simpler than I had hoped. I've added a small tweak which I think will help Windows, although not tested that yet (but probably will tomorrow).

[mdorman]

Ah, sure, I saw that code elsewhere, didn't quite clue into what it was doing.