Add option to configure Application channels

As discussed in #1321. Documentation still to come. 
+ I'm not sure what the difference between channel and channelName is for you? I would propose making channel the Radio channel

+ `getOption` would be great here, any good reason not to move line 19 up?

+ I'm thinking we should be passing channelName in and using that to get the app channel from Radio.

```js
this.channelName = this.getOption('channelName');
this.channel = Backbone.Wreqr.radio.channel(this.channelName);
this.vent...
this.commands...
this.reqres...
```