region.show() should respect view.onBeforeClose() return value

`Marionette.Region.show()` should respect the return value of `Marionette.View.onBeforeClose()`.  In particular, it should not close the region's current view, if that view's `onBeforeClose()` returns false, as [per spec](https://github.com/marionettejs/backbone.marionette/blob/master/docs/marionette.view.md#view-onbeforeclose).

related issue: https://github.com/marionettejs/backbone.marionette/issues/703