add route params to feathers params

similar to [feathers-associations/rest.js](https://github.com/feathersjs/feathers-associations/blob/master/lib/rest.js#L2-L6), i think it should be possible in core to

```
app.use('/groups/:groupID/members', groupMemberService)
```

and the group member service functions get groupID in params.

thoughts?
