Add an option to handle renamed commands

redis.conf :
  rename-command SET 807081f5afa96845a02816a28b7258c3
  rename-command GET f397808a43ceca3963e22b4e13deb672

client.js :
  var client = redis.createClient(6379, 'localhost', {
    commands_renamed: {
      set: '807081f5afa96845a02816a28b7258c3'
    }
  });

  // will return 'OK'
  client.set('key', 'value', function(err, reply) {
    console.log(err, reply);
  });

  // will return error : [Error: ERR unknown command 'get'] undefined
  client.get('key', function(err, reply) {
    console.log(err, reply);
  });