Implemented permanent user delete endpoint

Author: rhodgkins

README.md

@@ -147,6 +147,12 @@ client.users.find({ email: '[email protected]' }, callback);
 client.users.archive({ id: '1234' }, callback);
 ```
 
+```node
+// Permanently delete a user user by id (https://developers.intercom.com/v2.0/reference#delete-users)
+const intercomUserId = '123'
+client.users.requestPermanentDeletion(intercomUserId, callback);
+```
+
 ## Leads
 
 ```node

lib/user.js

@@ -43,4 +43,7 @@ export default class User {
   bulk(params, f) {
     return (new Bulk(this.client, 'user').bulk(params, f));
   }
+  requestPermanentDeletion(intercom_user_id, f) {
+    return this.client.post('/user_delete_requests', { intercom_user_id }, f);
+  }
 }