Allow non-DOM-backed regions with `allowMissingEl`

* `allowMissingEl` option is respected by `_ensureElement`
* `_ensureElement` returns boolean, indicating whether or not element is available
* `Region#show` early-terminates on missing element
* Increase jshint maxstatements allotment for `Region#show`
Merge pull request #2028 from duncanbeevers/region-allow-missing

Allow non-DOM-backed regions with `allowMissingEl`