Remove the TWITTER_HANDLE variable and use the 'twitter' value
from SOCIAL. This eliminates duplicate settings!
Remove closing slash for meta tags, not required by HTML5, see
http://stackoverflow.com/a/21898502
In article.html Twitter images were specified twice - once as
relative URL and once as full site URL. Now only a single full
URL is used instead. Also stripped HTML tags from description.
The default card type is summary_large_image which better displays
with high-res images and the default header images.
``twitter_image`` article metadata is now supported, similar to
``og_image``.
Documentation is updated.
