def comma(str, *phrases)
      _concat([", ", str])
      _inspect(phrases)
    end
Adds a comma then the provided phrase to this message.

Riot::Message.new.hello.comma("world").to_s
=> "hello, world"

@param [String] str any string phrase to be added after the comma
@param [Array<Object>] *phrases an array of objects to be inspected
@return [Riot::Message] this instance for use in chaining calls