Currently only works with 2 digits season and episode numbers. Also doesn't work
with multi-episode files.

The Markdown b:ml_commentstring was being overwritten because the
Markdown plugin includes the HTML plugin.

Also change the Markdown syntax override to include the ghmarkdown
syntax plugin instead of being a symlink to it.

Also deleted some FileType specific settings that aren't needed anymore.

It's always formatted the modeline as a line comment using the `commentstring`,
but now you can override the `commentstring` by setting `b:ml_commentstring` to
something else.