Add before:show events

Earlier today, jarrodpayne raised an issue in #marionette and stackOverflow [question](
https://stackoverflow.com/questions/22442198/can-i-render-views-to-layout-regions-before-showing-that-layout-in-a-parent-regi) about rendering a nested layout before its added to the DOM. 

I think a "before:show" event in `region.show` would be an elegant way to solve the problem. 

```js
var LayoutView = Marionette.Layout.extend({
    template: "#layout-template",
    regions: {
        one: ".one",
        two: ".two",
        three: ".three"
    },

    onBeforeShow: function() {
       this.one.show(new SubView());
        var col = new Backbone.Collection([{ p: 1}, {p: 2}, {p: 3}]);
        this.two.show(new CollectionView({
            collection: col
        }));
        this.three.show(new SubView());
    }
});
```

This way, `layout.el` would be fully rendered before `region.$el` is reset.