Support tilde code blocks

This is already supported by Atom's Markdown Preview, and GitHub.com itself. I'm personally interested in adding it because the Kramdown Markdown convertor used by Jekyll [only supports tilde fenced code blocks](http://kramdown.gettalong.org/syntax.html#language-of-code-blocks), and not backtick fenced code blocks.

Before
![before](https://cloud.githubusercontent.com/assets/1058778/3380370/647f8d12-fc04-11e3-8c22-ca480327e765.png)

After
![after](https://cloud.githubusercontent.com/assets/1058778/3380372/6d2b56ee-fc04-11e3-804e-6e63c2dfcfdd.png)