Nested attributes support

[flvrone]

Hey @janko-m,

I've just found another bug I believe, because I was trying to do exact same thing as you described here:
https://github.com/shrinerb/shrine/blob/master/doc/multiple_files.md
But with Mongoid (and currently supported embedded models on my branch), and well, it just skips all the Shrine uploaders stuff (kind of). It just saves file_data (if file attribute is just called file), but there's no uploading or validations. (I must use Mongoid 6.4 though).

[flvrone]

I've added tests for it, confirming that bug exists, but only for embedded models:
https://github.com/LinkUpStudioUA/shrine-mongoid/tree/nested-attributes

[flvrone]

OK, I've figured out that mongoid does not run callbacks for embedded models, and even cascade_callbacks association option does not help for some reason, not sure what's wrong there.
So I came up with a "workaround" test here: https://github.com/LinkUpStudioUA/shrine-mongoid/blob/nested-attributes/test/mongoid_test.rb#L280

So, if that's the only way to make it work - I believe such workaround should be described in README file.

Waiting for your review, @janko-m

[flvrone]

OK, this is actually bug in #swap method for embedded records, once it will be fixed,
cascade_callbacks association option will help.