. for multiple filetypes does not work with autocmd.

There seems to be no known workaround:
http://vi.stackexchange.com/questions/4893

This is a breaking change as it would break user scripts
who used autocmd, but we have already broken them
with this compound filetype, so let's just finish the job.

Semi-reverses: https://github.com/plasticboy/vim-markdown/pull/217

Signed-off-by: yuexin <yuecn41@gmail.com>