QoS overriding

[jacobperron]

#5 enables automatic QoS selection based on available publishers, but it would be nice to have the option to override the QoS used for a particular topic.

We could achieve this by adding to the TopicBridgeOptions class and exposing YAML configuration points.

In particular, it is already documented in the design doc that history and depth policies should be configurable since they can not be queried from publishers at runtime. Currently, they are defaulting to KEEP_LAST and 10.