Optionally serve directory listings

[jsdw]

Weave can serve static content, but it will not list the contents of some directory currently (instead, it looks for index.html if you request a directory and serves that, or fails if not found).

Adding the option to provide a directory listing would make weave useful in quickly sharing a bunch of content from your machine.

The main thing to think about here is how to add options specific to routes, especially as a bunch of potential features need them.