Allow configure repo_opts per query

[clouds56]

Now we could only configure prefix in repo_opts, is it possible to make the option also available in Dataloader.Ecto.load? For example

Dataloader.Ecto.load_many(Repo, :assoc_key, record,
    repo_opts: [prefix: "my_prefix"], other_option: true)

Now ecto-sql 3.0 already supported prefix well, hope dataloader could do better.

[agalvez-papa]

Please!

[benwilson512]

PR welcome for this.