Add support for UDP multicast

[jeremyroy]

Some sensors publish UDP data to multicast addresses (e.g. 224.0.2.2). It would be nice if the udp_driver could intercept multicast traffic and forward it to topic /udp_read.

Example implementation:
https://www.boost.org/doc/libs/1_35_0/doc/html/boost_asio/example/multicast/receiver.cpp

[JWhitleyWork]

Please consider creating a PR to add this feature.

[johannesn]

I created a pull request for this feature. If you still need this functionality, feel free to use my code.