Attr_List Extension now also supports nested ol's.

Not sure how I missed that when committing ea4af0d. Also as a side-effect,
this fixes #230 - which brought my previous oversight to my attention.
Thanks for the report @divisoryang.

Also added some tests - including tests of list items without attr_lists.
Sometimes I forget to test markup that does not use an extension when an
extension is enabled. That's what resulted in #230 being reported.