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.
@jfairbank - I'm with you on this. Besides cleaning up some ugly logic, it's actually a superior implementation.
https://github.com/marionettejs/backbone.marionette/blob/master/src/marionette.region.js#L61

In the current build (2.1.0) if you pass in improper configuration, there is no feedback. It simply sets the region to the passed object. I actually didn't realize there was an issue with my region until I tried to remove it.

If all agree I will update.