Add support for array fields

[rschef]

Add support for array fields, i.e.:

 schema "users" do
   field(:images, {:array, DummyDefinition.Type})
end

[ajacquierbret]

@achempion Would it be possible to review and merge this PR? I think it'd be a really nice to have feature!

[Jdyn]

I've tested the fork and it works. Would be great if it could be merged in.

[drselump14]

I would love to have this feature 🚀