Added support for post type archives in association field

[zevilz]

Added support for post type archives in association field.

Example:

Field::make( 'association', 'crb_association' )
    ->set_types( array(
        array(
            'type' => 'post_type_archive',
        ),
    ) )

Post type archives stored in database like below:
post_type_archive:post_type_archive:post_type_name