[gfm] Added strikethrough implementation

Resolves #1887. Added strikethrough implementation. I followed the code from strong since that seemed similar to strikethrough. Also @bfrohs was a major help!

There are some problems with my implementation that I would love to hear some feedback on to improve the pull request. 

  *  Tabs are not being picked up by the editor. Couldn't seem to write a test for it.
      ![strike tab](http://i.gyazo.com/65ceb88b3d67ec740e2ed9f631c0b239.png)
  *  The following test fails though I can't find the problem with my code.
      
      ```javascript
      MT("strikethroughWrapped",
        "[strikethrough ~~foo]",
        "[strikethrough  bar~~]");
      ```
  *  Problems when editing inside the strikethrough.
     
     *  Editing after the fist line of strikethrough removes it.
     ![strike tab](http://i.gyazo.com/87fff59b5fc8bdc8291d1acc5d272a33.gif)

     *  Editing on first line resets strikethrough to how I expect.
     ![strike tab](http://i.gyazo.com/66b2ac7a1120337990a2c90d20feea8e.gif)

     *  Joining lines doesn't add strikethrough unless first line.
     ![strike tab](http://i.gyazo.com/b533cad9afff5cd004a809c256ed7d56.gif)