Please add MultiMarkdown as a md parsing engine.

I created a tiny fork of Jekyll for the purposes of adding MultiMarkdown as my `.md` processor. The code mirrors other code in Jekyll as nearly as possible, though I'm not entirely sure that

    STDERR.puts '  $ [sudo] gem install multimarkdown'

is the best way to recommend getting the MultiMarkdown dependency. Fletcher Penney's [installation instructions](http://fletcherpenney.net/multimarkdown/install/) suggest other methods, and he has compiled installers.

One benefit, apart from having the markdown superset of my choice: by using the C version of MultiMarkdown (3.5), one thing I notice is how *fast* my projects compile.

Since many are using MMD these days, it would be lovely not to have to fork Jekyll to get these benefits.