Add: Table-level constraints

Syntax for table-level constraints:
 - __pk: field or [ fields ] for primary keys
 - __unique: field or [ fields ] for unicity
 - __check: <string> for table-level check

Now 'check' constraints are properly output for sql, with constraint names.

Now that a Pk can be declared, the <table>_id Pk is only generated as needed.

The yml example has been updated to not use reserved PGsql names (user, group)
=> (member, team)