[gfm] Adding support for ~~strikethrough~~ text

Fixes #1887.

> GFM adds syntax to strikethrough text, which is missing from standard Markdown.
> 
> ```
> ~~Mistaken text.~~
> ```
> becomes
> 
> ~~Mistaken text.~~

Added test cases, added a new style (strikethrough text is displayed with a strikethrough to be consistent with how the rest of the markdown is rendered).