Release 0.2.0

• Incompatible API change to cleanup some initial design decisions:
  the two level FormatOption/FormatMode model is removed, the
  fixed unit of FormatOption is moved to a new constructor
  FormatMode, and FormatOption is removed entirely. This should be
  a simpler API, at the cost of breaking compatibility.
• Fixed issue #3 (No support for negative numbers).
• Worked around issue #1 (Add 'base' unit) by adding a mode that
  disables scaling; it should have the same effect without introducing
  an artificial unit.