add hasView property to region

Fixes #1632

An alternative solution would be to declare `hasView` as a function and return a boolean based on whether `currentView` is set.

thoughts @marionettejs/marionette-core?
I don't think we even need a method for that. Most cases we can just test if currentView exists without explicit boolean casting.

```js
if (this.currentView) { ... }
this.currentView ? foo : bar;
```