Support `__format__`

[perhyps]

Hi, I think it would be very helpful to support the formatting of measures by adding the following definition to the MeasureBase class:

def __format__(self, format_spec):
    return '%s %s' % (
        format(getattr(self, self._default_unit), format_spec),
        self.unit
    )

This would allow the user to write something like this (e.g. in Python 3.6+):

>>> from measurement.measures import Distance
>>> x = Distance(cm=5/7)
>>> print(x)
0.7142857142857143 cm
>>> print(f"{x:.3f}")
0.714 cm