Added Tempo 'markup' property. NEW.

You can now use the markup=None keyword to cause a tempo to format as LilyPond
markup.

Example:

    >>> markup = Markup(r'\smaller \general-align #Y #DOWN \note-by-number #2 #0 #1 " = 67.5"')
    >>> tempo = Tempo(Duration(1, 4), 67.5, markup=markup)
    >>> staff = Staff("c'4 d'4 e'4 f'4")
    >>> score = Score([staff])
    >>> attach(tempo, staff)
    >>> show(score)

    >>> f(score)
    \new Score <<
        \new Staff {
            \tempo \markup {
            \smaller
                \general-align
                    #Y
                    #DOWN
                    \note-by-number
                        #2
                        #0
                        #1
            " = 67.5"
            }
            c'4
            d'4
            e'4
            f'4
        }
    >>

Note that this is the way LilyPond asks for unusual tempo indications (like
floats or fractions) to be handled.