trailing-whitespace extension doesn't allow for md.j2

error: bad --markdown-linebreak-ext extension '.md.j2' (has . / \ :)
  (probably filename; use '--markdown-linebreak-ext=EXT')```

[Natim]

would silently do the wrong thing for foo.fnd

Actually foo.fmd doesn’t match .md

[Natim]

~technically there’s no such thing as a double-extension:

>>> import os
>>> os.path.splitext('foo.md.j2')
('foo.md', '.j2')

If that’s a usecase we want to support, we could also use:

>>> filename = 'foo.md.j2'
>>> filename.endswith('.md.j2')
True