Audit attribute argument names and see if they can be made more consistent

[davidpdrsn]

I'm wondering if changing some of the attribute argument names to include "parent" or "child" would make them more consistent and easier to understand. It would also match the terminology used elsewhere in the API.

This would of course be a breaking change so I would to either deprecate the existing arguments or batch it together with https://github.com/davidpdrsn/juniper-eager-loading/issues/43