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.
Agree this is ugly. Not much better but I had:
```
var hasSelector = !!definition.selector;
var hasEl = !!definition.el;

if (isString || isObject && (hasSelector || hasEl) {

}
```

... which is slightly more readable but jshint threw errors.