Add option to start with a topic selected

[sea-bass]

For demo launching purposes, it would be nice to start rqt_py_trees with an optional argument that specifies the topic name to start subscribing to. For example, either or both of these:

rqt_py_trees --topic /my_node/log/tree
rosrun rqt_py_trees topic:=/my_node/log/tree

Otherwise, even if you make a launch file that automatically launches rqt_py_trees, users have to manually change the topic drop-down to the right topic.

The behavior could either be to wait for the desired topic to send a message, or if the topic isn't available to assume "none" and open rqt_py_trees as normal.