Parses command line options

def parse_options(args):
    """ Parses command line options """
    usage = 'usage: %prog [options] visio_filename image_filename'
    parser = OptionParser(usage=usage)
    parser.add_option('-p', '--page', action='store',
                      type='int', dest='pagenum',
                      help='pick a page by page number')
    parser.add_option('-n', '--name', action='store',
                      type='string', dest='pagename',
                      help='pick a page by page name')
    options, argv = parser.parse_args(args)

    if options.pagenum and options.pagename:
        parser.error('options --page and --name are mutually exclusive')

    if len(argv) != 2:
        parser.print_usage(sys.stderr)
        parser.exit()

    output_ext = os.path.splitext(argv[1])[1].lower()
    if output_ext not in ('.gif', '.jpg', '.png'):
        parser.error('Unsupported image format: %s' % argv[1])

    return options, argv