Protobuf 2 support

[mratsim]

The README specifies proto3 at the moment but does not say if proto2 will also be a target in the future.

So is it? Will you accept PR to extend to proto2?

[PMunch]

I'm not planning on writing proto2 support myself, but if anyone else wants to add proto2 support (and maintain it) I'd be all for it. IIRC the reason I went with just proto3 is because it was easier to parse. The current parser should support parsing it, but if you want you can rewrite the whole parser part, it is slow and breaks on larger files due to the recursion limit.