Feature: Support selecting the top level of a Custom API response

Some APIs return arrays at the top. With this change, a customapi widget
can select the top level object/array.

This is particularly useful when combined with the `size` formatter, but
can also be used for APIs that return scalar values directly.

docs/widgets/services/customapi.md

@@ -57,6 +57,9 @@ widget:
     - field: key
       label: Number of things in array
       format: size
+    - field: . # This will take the root of the API response, e.g. when APIs return an array
+      label: Number of items
+      format: size
 ```
 
 Supported formats for the values are `text`, `number`, `float`, `percent`, `bytes`, `bitrate`, `size`, `date` and `relativeDate`.