Draft of definition list buffer approach.
Definition list definitions are now pushed to context buffers, which are popped and formatted at the start of the next definition and at the end of list. This approach simplifies things (formatting is applied in one place, mddt_dlelement_end, instead of being spread across special cases in plain_text and list_end).

This breaks some formatted expected by existing tests, which now fail.

This method should be expanded to apply to all dl types (not just the plain def) as well as all (non-dl) list types.