Add support for eager_loading read_marks instead of joining

I had a need to load a bunch of read marks all at once in a query that used eager_load. with_read_marks doesn't work here because the other eager_load's override the fields so the read_marks.id is never selected.

This meets my needs but could probably use a little polish as I'm very new to active record / rails.

Tests passing:
18 tests, 58 assertions, 0 failures, 0 errors, 0 skips