feat(scully): allow specify headers when using the JSON plugin (#140)

user can specify a `headers` object in ths `scully.config.js`.
This headers will be passed throught the request when calling the
specified endpoint.

Closes #137

docs/plugins.md

@@ -88,7 +88,7 @@ exports.config = {
 ```
 
 The following is an example that uses the [jsonplaceholder](https://jsonplaceholder.typicode.com/) to fetch a list of
-User IDs for my app. It uses the `json` plugin.
+User IDs for my app. It uses the [JSON Plugin](../scully/routerPlugins/jsonRoutePlugin.ts) which is already part of Scully.
 
 ```javascript
 // scully.config.js
@@ -113,6 +113,26 @@ second is `property`.  The JSON plugin will pluck the provided property name fro
 means that the array returned by the jsonplaceholder api will each have an `id` property. So instead of returning a list
 users, it will return a list of userIds. 
 
+The JSON Plugin will optionally accept also a header configuration where you can set specific header that you may have to sent when requesting an API:
+
+```javascript
+// scully.config.js
+exports.config = {
+  routes: {
+    "/todos/:todoId": {
+      "type": "json",
+      "todoId": {
+        "url": "https://my-api.com/todos",
+        "property": "id",
+        "headers": {
+          "API-KEY": "0123456789"
+        }
+      }
+    }
+  }
+};
+```
+
 ### <a name="router-plugin-configure"></a> Router Plugin Examples
 For those looking to build router plugins for their app, here are links to the built-in __router plugins__ in Scully:
 - [JSON Plugin](../scully/routerPlugins/jsonRoutePlugin.ts)