UX: Add theme screenshots (#61)

Soon we will have a mechanism in core to show
screenshots defined in a theme or component's
repo when that theme is installed. Air is a good
theme to start this with for testing.

Author: martin-brennan

about.json

@@ -35,5 +35,6 @@
       "success": "71bd9f",
       "love": "c16ad7"
     }
-  }
+  },
+  "screenshots": ["screenshots/light.png", "screenshots/dark.png"]
 }