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: Idc0d75a7ceab7c8ce460ef896fdefdec088124bb