Structure

This is changeset attempts to allow DDG::ZeroClickInfo to contain a structured result.  The contents of that result are, as yet, undefined, but a merge is necssary to proceed on further goodies template work.

- Add a "structured" zci element to signal intent to return a structured result.
- Allow the return of the `structured_result` attribute
- Add the attribute in the `IsStructurable` role.

This is not my favorite way to do this, but it confroms with the general code layout and style to make progress for now.

//cc @russellholt