Add Easy Digital Downloads license option

Here is an example of how to register the option:

    array(
        'name'      => __( 'License Key', 'wpbo' ),
        'id'        => 'mc_license_key',
        'type'      => 'edd-license',
        'default'   => '',
        'server'    => esc_url( 'http://myshop.com' ),
        'item_name' => 'My Product',
    )