add support for covering index (INCLUDE) on PG producer

[rabbiveesh]

This adds support for covering indices (i.e. the "INCLUDE" clause on create index) for the Postgres producer.

the intended interface is as follows

$table->add_index(
   name => 'covering', fields => [ 'foo' ], 
   options => { include => [ 'bar', 'baz' ] }
);

In the current implementation it silently skips the clause if PG is under
version 10, b/c it was only added in 11 (as far as I remember). I'm not quite
sure if it should be that way, or if it should die.

[rabbiveesh]

The only thing left here is to improve the docs with the updated index grammar.