Guarantee uniqueness for columns

[armenak]

An additional option to guarantee one value isn't reused - for instance if email column is set as a UNIQUE index for a table.

This may be an issue if the number of possible values is less than the number of records in a table. The solution should (eventually, perhaps initially it's enough to require a list of possibilities large enough) have a way of guaranteeing uniqueness if a value has already been used perhaps with the addition of a 'Xeger' pattern.

[armenak]

Duplicate of issue #8