Add `find` method to `ReadModel`

This adds a `find` method to `ReadModel`. This will allow querying
for a specific record in the collection. It will use
ActiveRecord's `find` method on the relation. The relation itself
is only built from a subset of the read model's expressions. It
will not use any `where`, `order`, `limit` or `offset` expressions.
Furthermore, it will use ActiveRecord's `except` to try and remove
any `where`, `order`, `limit` or `offset` expressions that got
applied, possibly through a generic `merge`. This is because
`order` doesn't matter when selecting a specific record and
`where`, `limit` and `offset` can remove the record from the
results.

Closes #142