Extended Tremolo with 'is_slurred' property.

Set to true to slur tremolo:

    >>> chord = Chord("<cs' e'>4")
    >>> tremolo = indicatortools.Tremolo(is_slurred=True)
    >>> attach(tremolo, chord)

    >>> f(chord)
    \repeat tremolo 4
    {
    cs'32 \( e'32 \)
    }

This is the correct notation for fingered tremolo on bowed string instruments.

Defaults to false.