Fire an action when order limits are active

[stevegrunwell]

When OrderLimiter::disable_ordering() is called, an action should be fired to let other plugins get in on the action.

/**
 * Fires when ordering has been disabled.
 *
 * @param \Nexcess\LimitOrders\OrderLimiter $limiter The order limiter instance.
 */
do_action( 'limit_orders_disable_ordering', $this );

[stevegrunwell]

It may also make sense to tie our default disabling actions to this hook, which would make it easier for store owners to say "actually, don't disable if X conditions are met".