Adding ability to pass config/data to ViewEngine

The desire here is to be able to pass data into the __layout.hbs when it is initially rendered on the server. It's data that we don't want to go into the actual app as it's too big and only needs to be generated once when the server starts up (eg: inline css).

Initially I'd created our own ViewEngine and was passing that into the server, but this seemed like such a small change, and that this.options hash was just forlornly sitting there begging to be used.

Using `renderData` feels a little cludgy, totally open to better naming, something more dynamic.