Merge branch '55-twitter-cards' into '2.2-dev'

Add twitter metadata

See merge request !93
4 jobs for 111-explicit-relative in 1 minute and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #2633
Go-py2.7

00:01:36

passed #2634
Go-py3.4

00:01:38

passed #2635
Go-py3.5

00:01:37

passed #2636
Go-py3.6

00:01:37