Add --hide-error-context back (undocumented)

[gvanrossum]

See PR #2540. This replaced --hide-error-context with --show-error-context (and changed the default). But for backwards compatibility and in case one wants to override the config file with a manual flag it would be nice to add

parser.add_argument('--hide-error-context', action='store_true',
                        dest='hide_error_context',
                        help=argparse.SUPPRESS)

[gvanrossum]

While you're at it maybe move the hide_error_context option out of the experimental section in options.py?