Allow the relationships to lazy load meta_data.

[baash05]

I'd love to be able to selectively run logic in the meta only if the relationship is included.
So I could avoid calling the Posts.count method if posts weren't included.
It would also allow me to use Posts.size and perhaps take advantage of eager loading.

[beauby]

Thanks for your feedback – I do not have time to add new features at the moment but I'll keep this issue open in case someone else wants to tackle it in the mean time.

[beauby]

Ref #101.