JSON format modernisation and _shields_test removal (#3272)

* Modernised JSON format and removed _shields_test style

* Added logoWidth and labelColor fields to JSON response

* Reinstated and updated comment

* Extended expectBadge to accept Joi schemas for all fields

doc/json-format.md

@@ -0,0 +1,25 @@
+# JSON Format
+
+Even though Shields is probably best known for its SVG badges, you can also retrieve
+a JSON payload by replacing the `.svg` extension with `.json`.
+
+For instance, hitting [this endpoint](https://img.shields.io/badge/hello-world-brightgreen.json)
+will generate the following payload:
+
+```
+{
+  "name": "hello",
+  "label": "hello",
+  "value": "world",
+  "message": "world",
+  "color": "brightgreen"
+}
+```
+
+Note that the values of the `name` and `value` fields are duplicates of the `label`
+and `message` ones, respectively. As of April 2019, `name` and `value` are deprecated
+and will be removed in a future release, please consider migrating your application
+to use `label` and `message` instead.
+
+Feel free to [open an issue](https://github.com/badges/shields/issues/new/choose)
+if you have any queries regarding the JSON format.