Extended Tempo with _to_markup() method.

Example:

    >>> tempo = Tempo(Duration(1, 4), 60)
    >>> markup = tempo._to_markup()
    >>> print(format(markup))
    \markup {
            \smaller
                    \general-align
                            #Y
                            #DOWN
                            \note-by-number
                                    #2
                                    #0
                                    #1
            " = 60"
            }