Automatically add foreign key constraints

[cycomachead]

It would be awesome if when using a type foreign_key, Lapis could automatically create a new foreign key constraint in the DB.

The would mean Lapis would need to allow type.foreign_key to accept some new arguments, like { table = 'users' } in order to know what the referenced table should be. However, a default could work such that any column of the form {other_model}_id would use that table, à la Rails and other ORMs.

However, it is worth noting that adding "FOREIGN KEY (user_id) REFERENCES users" (for example) to the create_table call accomplishes the same result. So it's not a huge deal. :)