Add test for new use_post_dirs configuration option

Also add a use_post_dirs attribute to Site and the default
configuration.

When the permalink format doesn't include html:

- If use_post_dirs is set (the default) make a directory with that
  permalink and put the file as index.html.
- Otherwise, just append .html, and assume that generated URLs will be
  rewritten to point to it correctly.
Implement use_post_dirs option

Read its value from the config (if any) and use it in Post.destination.
Make server redirect to .html if use_post_dirs is not set