Allow directory listing in serve mode.

Just a small tweak to allow directory listings when running `serve`, particularly `serve -w` for local development.

One of my colleagues had an issue where there was a newline at the beginning of an `index.md` file, causing it to be built as `index.md` rather than `index.html`, resulting in a very confusing "access forbidden" error message when viewing the expected path. We only use `serve -w` for local development; the deployed site is served by Apache, so with this patch, we can just set `dirlist` in our `_config.yml` file, and it will be much more obvious when little problems like messed up front matter prevent index files from building.