Add Alt text support for images.

[atomicules]

As per these announcements:

• https://blog.twitter.com/2016/accessible-images-for-everyone
• https://blog.twitter.com/2016/alt-text-support-for-twitter-cards-and-the-rest-api

Getting it via json is easy and just a matter of including a url parameter. However, we need to think a little about how it is displayed, especially since it can be up to 420 characters per images.