Added strikethrough syntax to GitHub Flavored Markdown parser

[diegobg]

GitHub Flavored Markdown supports strikethrough text according to GitHub Help - 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 <del> HTML element that contains the text. This doesn't affect the standard kramdown parser.

[gettalong]

Does the code work correctly for the following case:

This ~~is not~~~ possible.

I.e. what happens to the extra tilde?

[gettalong]

Lines 36 and 40 in gfm.rb are modified for whitespace only, please revert this.

[gettalong]

One more thing: There is no syntax description for the strikethrough syntax on Github, so please test on github some more variations and add tests for them - thanks!

Otherwise it is looking good!

[gettalong]

Please fix the problems and I will re-open.

[tkrotoff]

+1 for this feature

[CGarces]

Is strikethrough supported?
After the migration of github pages to kramdown I have lost my formatting

~~This~~ no longer works

I have done a sample page with the issue.