Allow custom metadata to be persisted on Launch Resources via API

As a Launch API user, I would love to have place to attach my own metadata to a Launch resource when creating/updating it. 

As an example, I might want to create a data element by posting to the standard endpoint,  POST /properties/:property_id/data_elements

In addition to the normal, required parameters, I'd like to have a place to store my own metadata (perhaps a stringified JSON object up to some sane size limit).

This would allow for any number of expanded uses.

Thanks for the consideration.

-Stew