Add support for URL icons

[gripfastistech]

add icon images before site titles in static site: Originally posted by @gripfastistech in #70 (comment)

I think it's just easier to add an icon feature for each site rather than trying to add an <img... in the YAML site title. Can you open a separate issue for "Add support for URL icons"? Then we can show these icons in the README and on the status website.

Originally posted by @AnandChowdhary in #70 (comment)

[AnandChowdhary]

Custom icon support has been added to the README.md file, and will soon be added to the status page website as well. You can add a custom icons like this:

sites:
  name: ...
  url: ...
  icon: https://example.com/favicon.png

If you don't provide an icon, we'll show you the icon using GitHub's favicon service with a fallback to a globe icon:

[AnandChowdhary]

Icons are also available on the status site now: