Support capturing colspan and rowspan on table elements

[chriscarrollsmith]

The colspan and rowspan attributes are pretty important for capturing structural information about tables.

• We could add support for capturing arbitrary attributes, but,
• Alternatively, we could make table the leaf node and treat its internal row/cell tags as plaintext. That would reduce some complexity related to processing, storing, displaying, embedding, and querying tables.

I think the second option is ultimately what we should move toward.