Allow to specify a type for `references` in migrations

This allows to specify a type for created foreign key column in `references` and `add_reference` in migrations, very handy:

```ruby
def change
  enable_extension 'uuid-ossp'
  create_table :cars, id: :uuid do |t|
    t.integer :seats
    # And other car-specific things
  end
  create_table :wheels do |t|
    t.references :car, type: :uuid, index: true
    t.integer :radius
    # And other wheel-specific things
  end
end
```

Also I've changed PostgreSQL test to use this shiny functionality (looks better, eh?). And if anyone will ever remove this (not by reverting this PR), test will fail, showing that folks that use Postgres really need it ;-)

Bonus: documentation for `references` method.

P.S> After I wrote this PR I've found PR #13959 (which does the same, but little bit lengthy). Also see it for previous discussion. Choose either you like, I'll be happy with any of these PRs merged.

Will close #13959 if merged.
No, just followed other tests in this file…