Add FactoryGirl integration

Introduce optional FactoryGirl syntax that makes it easier to use Formulaic
to fill in attributes from a factory with a single argument:
`fill_with_factory(:user)`; limit the attributes from a factory:
`fill_with_factory(:user, :name, :address)`; and even pass arguments to the
factory, such as traits and overrides:
`fill_with_factory(:user, :name, :address, factory_arguments: [:with_verified_address]`.