Add StackOverflow social icon and support for Disqus post comments

This adds a social icon for StackOverflow (using Font Awesome's icon), where the user can specify their StackOverflow user ID in `_config.yml`.

This also adds optional support for Disqus comments. The comment section goes well with the Pixyll style - the colour and font all looks correct to me - and it's disabled by leaving the short name blank in `_config.yml` by default.