Added strikethrough syntax to GitHub Flavored Markdown parser

GitHub Flavored Markdown supports strikethrough text according to [GitHub Help - GitHub Flavored Markdown](https://help.github.com/articles/github-flavored-markdown/) but the GFM parser included with kramdown doesn't support it.

I've added strikethrough support to the GFM parser: when it encounters text delimited by two or more tildes, it creates a &lt;del&gt; HTML element that contains the text. This doesn't affect the standard kramdown parser.