Filtering on pivot table

[atymic]

Hey :)

I have a case where we'd like to filter on some values in a pivot table, however I can't see a way to do so with the library.

For example:

A book can be loaned to many people, so we have a book_person table a due_date property.

Normally, you can just do wherePivot(). Do you think would make sense in this library?
If so, would you accept a PR?

Thanks!