Autoload csv files from data directory

Sometimes it's simplest to store data in CSV format. This PR autoloads these files as well, just like JSON or YAML.
Hashrocket added.

As for headers, if you didn't have headers in the CSV, there would be no way to do things like site.members.name (as there wouldn't be anything to say it was a name), so I think it's OK for Jekyll to support a very precise definitions of CSV, i.e. comma separated and includes header row. That's what most people will want to use anyway. If there wasn't a header row, you'd get junk data, but there's currently no simple way to be sure if a CSV has a header or not, so we can't really throw an error.