Slide separator dictionary now serves mkd markdown

Since Plastic Boy's Vim Markdown plugin (and few others) work with
filetype `mkd`, they set up filetype so on following events:

* buffer read
* buffer new

Both causes the slide dictionary to return "no such key" since it has
"markdown" key, not "mkd".

This is the real reason behind issue I faced and tried to solve
in commits (7bb6198345b and b045e9842).

Note that with the slides dictionary now serving both `markdown` and
`mkd` files, the clause that previous commits highlighted may well be
unnecessary now, even with said other plugins equipped.

Computers are getting smarter all the time.  Scientists tell us that soon they will be able to talk to us. And by ‘they’, I mean ‘computers’.  I doubt scientists will ever be able to talk to us.
		-- Dave Barry