I wonder if we should do `... in (True, 'true', 't', 1, '1', 'yes', 'y')`. Some people might create an instance in Python and pass it into Markdown. In that case, we definitely need to account for the bool value `True`. And doesn't the command line pass a string? If so, we need to accept the string `'1'`. The values `'t'`, `'yes'` and `'y'` are probably not necessary, but nice to have (especially with command line users - not so much with pythonists).