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".

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
Merge pull request #8 from LIttleAncientForestKami/greateMarkdownSupport

Slide separator dictionary now serves mkd markdown