Parse pretty-formatted quantity expressions.

Currently pretty-formatted (using 'P' format spec) quantities cannot be
parsed by UnitRegistry. This allows, for example:

  >>> q = ureg['meter ** 0.5 / second ** 2']
  >>> pretty = format(q, 'P~')
  >>> pretty
  '1.0 m⁰.⁵/s²'
  >>> q == ureg[pretty]
  True

Unit tests have been added to test this feature.