code: public CommandBuilder addArg(String opt, Object arg) {
      mArgs.add(opt + " " + String.valueOf(arg));
      return this;
    }
docs: Adds the string value of the given option argument to the command.
  
  For example, to add "-name myFile" to construct the command "find . -name myFile", opt should
  be "-name" and arg should be "myFile".
  
  @param opt the option flag
  @param arg the argument
  @return the {@link CommandBuilder} with the argument added