Add Support for Rosout

[netzdoktor]

For testing purposes, it would be great if the logging could be dynamically configures to publish to the /rosout topic, as in the roscpp and rospy modules.

[neachdainn]

I would think that doing this could easily be the responsibility of a rosrust-log crate. Implementing a Rust logger looks fairly simple, so from there it's just a matter of forwarding it to rosout.