add plugin annotation

[u2takey]

this is a proposal and implementation for a new plugin annotation, which add an string annotation for message type, which can be used like JAVA's runtime annotation.

Usage:

message X {
    option (gogoproto.annotation) = 'a string';
}

message Y {
    // annotation could be a json string
    option (gogoproto.annotation) = '{"A":"a"}';
}

Retrieve

import (
    gogoproto "github.com/gogo/protobuf/proto"
)

fmt.Println(gogoproto.GetAnnotations())
fmt.Println(gogoproto.GetAnnotation(&proto.X{}))

[u2takey]

@awalterschulze @jmarais