tpope/vim-markdown — pull request #48 by muff1nman (2013-08-06 00:53:00+00:00)
Title: Recognize ~~~ the same way ``` is recognized

I'm not sure if you are okay with brining in syntax rules of markdown derivatives, but I thought I'd try as this plugin is great with [kramdown](http://kramdown.rubyforge.org/syntax.html#code-blocks).