- Added a register_optionflag function (so users can add their own   option flags); and use it to define the existing optionflag   constants.
- Added a register_optionflag function (so users can add their own
  option flags); and use it to define the existing optionflag
  constants.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator