allowMissing option permits non-DOM-backed regions

We use a Layout parent class to capture behavior common to a number of subclass views.

There are behaviors that drive regions which exist in some of the subclasses and not in others. Prior to #1024 / #1050 these missing regions would simply act as no-ops in these subclass views, but now they throw errors.

In order to accommodate this use-case, I added an `allowMissing` option to be passed in through `show`, which suppresses the error-throwing behavior in `_ensureElement`, and turns `appendHtml` into a no-op in the case of a missing `this`.el`.