Validate extra context.

def validate_extra_context(ctx, param, value):
    """Validate extra context."""
    for s in value:
        if '=' not in s:
            raise click.BadParameter(
                'EXTRA_CONTEXT should contain items of the form key=value; '
                "'{}' doesn't match that form".format(s)
            )

    # Convert tuple -- e.g.: (u'program_name=foobar', u'startsecs=66')
    # to dict -- e.g.: {'program_name': 'foobar', 'startsecs': '66'}
    return collections.OrderedDict(s.split('=', 1) for s in value) or None