[feature]Support variables substitutions (of form $var/${var}) in relay module.

[ever4Kenny]

Support variables substitutions (of form $var/${var}) in RELAY module:
pull rtmp://xxx.com/live/$name?$args;
Just like the EXEC module:
exec_play echo rtmp://xxx.com/live/$name?$args >>/var/log/rtmp-$name.log;