Pass a function to define regions with RegionManager#addRegions

Implements #1371.
This is testing when the custom region doesn't have `el` set on it because it will be provided later in the region definition [here](https://github.com/marionettejs/backbone.marionette/pull/1621/files#diff-6e0d4d8ca072dbae6d5bcc7e1184a142R330).