Add line at the end of file
Change check for article.modified which is no longer needed after change in Pelican (>3.3) code

See these discussions

https://github.com/getpelican/pelican/pull/1161
https://github.com/getpelican/pelican/pull/1148
Add TWITTER_USERNAME and GOOGLE_PLUS_PROFILE_URL in _defaults

It will be used for social SEO.

Update #15
Add Google Authorship code to article and page

Update #15
Add social SEO and Google Authorship code to index

Update #15
Add SEO metadata to article and page

Update #15