Add options for foreign key attributes

[guzman-raphael]

Fix #110

• Adds unique option for primary and nonprimary foreign key attributes
• Adds nullable option for nonprimary foreign key attributes

For instance, the syntax to properly set within the Table definition could be:

%{
# ActiveSession
-> [unique] Lab.Subject
---
(session_rig_class, session_rig_id) -> [nullable, unique] Lab.Rig(rig_manufacturer, rig_model)
%}

Note: describe currently does not support showing set indices and will continue to not show with this update. Will file a separate issue to track it.