Add emitting default values support in JSON

[wish-master]

It will be great if I can paste some flag to generator which pass EmitDefaults value in default marshaler. This option is well discussed in that thread: grpc-ecosystem/grpc-gateway#233
I suppose to make it optional with false as default value not to change current behavior.
I'm talking about flag exactly, not to develop some custom httpruntime.Marshaler implementation and injection... If @utrack will be agree, I'll implement this feature.

[bullgare]

I'd be interested in this feature as well.

[utrack]

It's a great idea, I believe :) I'm all for it if you'll manage to override marshaler settings per-handler, without breaking global injection.