[Discussion] #include method on base collection

Throwing this out there for discussion...  I'm going to need to make *a lot* of API calls to get the view_counts and such of videos, and I'd like a way to do that as efficiently as possible (both time and quota).

Each call to the API is one additional quota unit, as well as ~300ms waiting.

What do you think about adding an `include` method to specify additional parts?

```ruby
Yt::Channel.new(id: '').videos.include(:statistics).where(channel_id: nil, id: 'K9XCKP9KN7A').first
```