json_error_handler: cfg.includeErrStack

Support flag for development and integration environments: includeErrStack
```
    openapi-error: 
      name:                 json_error_handler
      handle500Errors:      true
      includeErrStack:        true

```

Change is backward compatible (missing config is understood as the default - false)

I still need to add tests

Author: osher

fittings/json_error_handler.js

@@ -35,6 +35,8 @@ module.exports = function create(fittingDef, bagpipes) {
 
       context.headers['Content-Type'] = 'application/json';
       Object.defineProperty(err, 'message', { enumerable: true }); // include message property in response
+      if (fittingDef.includeErrStack)
+          Object.defineProperty(err, 'stack', { enumerable: true }); // include stack property in response
 
       delete(context.error);
       next(null, JSON.stringify(err));