Add template name param to renderPage helper

[tomconroy]

For use mainly in the meteor-transitioner project. but may have other applications. For example:

{{renderPage currentScreen}}

or

[[renderPage "news"}}

(second one untested)

The checking of template name is a little hairy. Not sure the best way of checking if something is passed in?

[tmeasday]

Thanks Tom! Yeah, I checked in a slightly different way, much appreciated.