specify include path to resolve imported file

[umegaya]

this pull request aims to add functionality to specify include path to resolve imported proto file, stated in #368

include path can be specified via Builder object like following:

var builder = ProtoBuf.newBuilder({
    includePath:[
        './proto'
    ]
});
var protoBuilder = ProtoBuf.loadProtoFile('./proto/foo.proto', builder);

I know you don't want to support resolving path so much, but its kind of "nice to have" feature, right?

[aidansteele]

This would be very useful for the pbjs CLI tool. I would like to be able to pass in multiple paths, e.g. pbjs -p . -p /usr/local/opt/protobuf/include file.proto and have it able to import both my proto definitions and Google's definitions bundled with protoc.

[umegaya]

@aidansteele thank you for your suggestion! but it means all .proto files need to convert to .json file once, and program loads them rite? if so, I think loading .proto directly is more handy way, if purpose is running tool. especially we have a lot of .proto files like this. I wonder why protobuf.js itself does not support this.