Allow multiple terms to be associated with a labeled list item in the model

[mojavelinux]

The model for labeled lists does not allow for multiple terms to be associated with a single definition. As a result, the backends have to do some nasty logic to detect orphan terms and link them in the markup to the next term that has a definition. While this logic is possible in ERB, it's very difficult to implement in Haml and Slim.

The correct way to address this issue is to store the multiple terms together in the model so that rendering is straightforward...and the model communicates the proper semantics.

Allow multiple terms to be associated with a single labeled list item in the model.