Add ability to delete all markers in Marker plugin

This simple change allows you to publish a message to Rviz to clear all markers in a particular Marker display via the provided ROS message topic.

To clear all, use the regular ``visualization_msgs::Marker::DELETE`` action, but leave the ``id=0`` and ``namespace=""`` (empty). This would be *very* useful to me for restarting my application and clearing out all the previous left over markers. I'm always having to click that Reset button.