add voip_carriers.e164_leading_plus for carrier-level configuration of E.164 dial string

lib/models/voip-carrier.js

@@ -21,6 +21,18 @@ VoipCarrier.fields = [
   {
     name: 'description',
     type: 'string'
+  },
+  {
+    name: 'account_sid',
+    type: 'string',
+  },
+  {
+    name: 'application_sid',
+    type: 'string'
+  },
+  {
+    name: 'e164_leading_plus',
+    type: 'number'
   }
 ];