Allow use of `el` in RegionManager.addRegion definition.

From the [Docs](http://marionettejs.com/docs/marionette.region.html):

> Finally, you can define regions with an object literal. Object literal definitions normally expect a selector or el property. The selector property is a selector string, and the el property can be a selector string, a jQuery object, or an HTML node.

RegionManager does not currently support region definitions that use `el`. As LayoutView uses RegionManager behind the scenes, it also will not be able to use `el` when defining region definitions.