def run_definition(defin, &block)
      command = defin
      defin.instance_eval(&block) if block
      add_command(command)
    end
def run_definition(defin, &block)
      command = defin
      defin.instance_eval(&block) if block
      add_command(command)
    end
default rake task initializer
 call this to run a {CommandDefinition} subclass
 @param defin an instance of a command definition subclass
 @yield in the block, you can modify the internal state of the command,
   using desc, add_filter, etc.