Add option to configure Application channels

As discussed in #1321. Documentation still to come. 
I should be more specific, this should test the exact logic happening in `_initChannel`.

#### When `channelName` is specified 

```js
this.channelName = 'foo';
this.App = Marionette.Application.extend({ channelName: 'foo' });
this.app = new this.App();
this.channel = Backbone.Wreqr.radio.channel(this.channelName);
```

1. It should create a Wreqr channel on `this.channel` 
  - expect `this.app.channel` to deep equal `this.channel`
2. It should set the channels EventAggregator to `this.vent`
  - expect `this.app.vent` to deep equal `this.channel.vent`
3. It should set the channels EventAggregator to `this.vent`
  - expect `this.app.vent` to deep equal `this.channel.vent`
4. It should set the channels Commands to `this.commands`
  - expect `this.app.commands` to deep equal `this.channel.commands`
5. It should set the channels Commands to `this.reqres`
  - expect `this.app.reqres` to deep equal `this.channel.reqres`

#### When `channelName` is specified as a function

```js
this.channelName = 'foo';
this.channelNameStub = this.sinon.stub().returns(this.channelName);
this.App = Marionette.Application.extend({
  channelName: this.channelNameStub
});
this.app = new this.App();
this.channel = Backbone.Wreqr.radio.channel(this.channelName);
```

1. It should set `this.channelName` to the result of `channelName`
  - expect `this.app.channelName` to equal `this.channelName`

#### When no `channelName` is specified

```js
this.app = new Marionette.Application();
this.channel = Backbone.Wreqr.radio.channel(this.channelName);
```

1. It should set `this.channelName` to `"global"`
  - expect `this.app.channelName` to equal `"global"`

Alright... so I went a little overboard, but yeah that's what it should be.