[Feature Request] api-key property for google-client-loader

[Jookus]

For projects with a custom API on google cloud endpoints it would be nice if there was an api-key property in google-client-loader. That way I wouldn't have to set it myself through gapi.client.setApiKey("###API-KEY###"); when gapi.client has been loaded.

<google-client-loader id="myApi"
                         app-id="my-appengine-project"
                         name="myapi"
                         version="v1"
                         api-key="###API-KEY###">
   </google-client-loader>