Add option to configure Application channels

As discussed in #1321. Documentation still to come. 
Sure thing. I think I chose bad variable names for my example. I was just trying to demonstrate that properties from options would not be available when resolving channel.

trying for a better example (using `channel` to be consistent with current code):
```js
      this.app = new Marionette.Application({
        someArbitraryProperty: 'foo',
        channel: function () {
          return this.someArbitraryProperty; // returns undefined
        }
      });
```