Add support for syntax tag in .proto

[lpgauth]

Hi,
I'm currently experimenting with protobuf 3.0. It would be nice to add support for syntax = "proto2"; or syntax = "proto3"; to specify a syntax version. This is part of protobuf 3.0-beta2.

Cheers,
LP

[tomas-abrahamsson]

Good point. Support for proto3 is still mostly to do, but in any case, the syntax statement ought to be supported, so I added support for it in 3.20.2. However, it only recognizes "proto2" so far. For now, specifying "proto3" will cause a parse time error, until more proto3 support is in place.

[lpgauth]

Perfect, thanks!