SSM can be enabled using "source" option in UDP options. Previously, "source" is considered the same as "interface", which is incorrect. Fixes #332.
It aligns with what a lot of other apps are doing, e.g. tsplay. Also added notes in docker to adjust network setting if wants to use multicast in docker container. Change-Id: I666a8979cc041c904f7ff8e3e022f800c0830d5d
Change-Id: Ia043612012ff3d101afcf3366d95f9001f0dc8f4