Add option to configure Application channels

As discussed in #1321. Documentation still to come. 
Okay @cmaher, at the end of the Application class would you mind adding:

```diff
   triggerMethod: Marionette.triggerMethod

+  // Proxy `getOption` to enable getting options from this or this.options by name.
+  getOption: Marionette.proxyGetOption
```

And then use this instead:
```diff
-  this.channelName = _.result(this, 'channelName') || 'global';
+  this.channelName = this.getOption('channelName') || 'global';
```