Fixture prop

Currently lookup tables lets you create properties on individual columns of a Table. This implementation allows to add properties (XML attributes) on the entire row of a table (attribute on a higher level XML element).

Implemented as specified in the [spec](https://docs.google.com/a/dimagi.com/document/d/1yVSotTa33_UF9dd5iuRYc80FbkqP6sIWGiiPaAP_gVQ/edit#heading=h.v1gf9mhs7nhi).
two very-tiny, non-actionable stylistic things:
1. not required, but always nice to map params to the order in the function signature (in this case since you went out of the way to put it in the middle of the params it seems odd to then put this at the end).
2. if you put a trailing comma on the last argument then you won't get this multi-line diff next time someone adds a param