code: function execute(log, command, opt_callback) {
    if (!registry[command]) {
      registry[command] = new Executable(log, command)
    }
  
    var executable = registry[command]
    if (opt_callback) executable.addCallback(opt_callback)
  }
docs: Executes a memoized command and triggers the callback on a result.
  @param {Log} log
  @param {string} command
  @param {function(Error, Buffer, Buffer)=} opt_callback