Support grpc extensions

[daurnimator]

gRPC extends the .proto file format to add service+rpc sections.

[daurnimator]

For the grpc hello world example https://github.com/grpc/grpc/blob/6eae794c9f8f305a51274a79400124093ddc9354/examples/protos/helloworld.proto
I get:

lua5.1: Lexical error in line 27, near "lloReply) {": expected ';'
stack traceback:
	[C]: in function 'error'
	/usr/share/lua/5.1/pb/proto/scanner.lua:77: in function </usr/share/lua/5.1/pb/proto/scanner.lua:75>
	[C]: in function 'match'
	/usr/share/lua/5.1/pb/proto/parser.lua:301: in function 'parse'
	/usr/share/lua/5.1/pb.lua:199: in function </usr/share/lua/5.1/pb.lua:193>
	(tail call): ?