Add option to configure Application channels

As discussed in #1321. Documentation still to come. 
Makes sense that you might want to pass in the channel directly or as a function.


I see a series of optional assignments: get channel unless passed in, get vent|reqres|commands unless passed in. Am i missing something? 


```js
_.extend(this, options);
this.channelName = _.result(this, 'channelName') || 'global'
this.channel = _.result(this, 'channel') || Backbone.Wreqr.radio.channel(this.channelName);
this.vent = _.result(this, 'vent') || ...
this.commands...
this.reqres...
```