Add config option to enable waiting by default on all content elements

On my current project our CI server is running many builds in parallel and often gets overloaded, which slows down our Geb tests. To guard against timing failures due to slow page loads on our CI server, I've had to add (wait: true) to all of our page object elements. 

Rather than needing to enable waiting on each individual page object element, I wanted to be able to enable waiting by default via a config value in the GebConfig file.