Add ability to render helpers with arbitrary data.

I'm trying to render the contents of a loop based on values from a database, but using a lambda so that I can do it lazily (ie not call the database unless the template contains certain tags).

Just picture that the line:

    $people = array(

is actually a database call.

Feel free to push back if you feel like there's a better way to do this, but I've been running this change in production for a while and it works well.

Thanks!