title: option "class_prefix" unknown metasyntactic
body: <p>I am using <a href="https://code.google.com/p/metasyntactic/wiki/ProtocolBuffers" rel="nofollow">metasyntactic</a> to convert my protobuf to objective C. I am trying to use the extension for adding class prefix. My code looks like</p>
  
  <pre><code>import "google/protobuf/objectivec-descriptor.proto";
  
  option (google.protobuf.objectivec_file_options).class_prefix = "TS";
  
  message Test {
    optional string s1 = 1;
    optional string s2 = 2;
  }
  </code></pre>
  
  <p>But when I try to compile it, I get the error: </p>
  
  <pre><code>Option "(google.protobuf.objectivec_file_options)" unknown
  </code></pre>
  
  <p>Can someone tell me what am I doing wrong?</p>