Support for oneof custom options

[kshchepanovskyi]

Sample:

syntax = "proto2";

package protostuff_unittest;

import "google/protobuf/descriptor.proto";

extend google.protobuf.OneofOptions {
    optional int32 foo = 50003;
}

message SampleMessage {
    oneof test_oneof {
        option (foo) = -99;
        string name = 4;
    }
}