Data as a function

With this change render task support builder function for data option. May be useful to make grunt runtime operations on templating scope, like this:

    render: {
       index: {
            options: {
                data: function () {
                    return { 
                        config: getConfig() 
                    };
                }
            },
            files: {
                'index.html': 'index.ejs'
            }
        }
    }