FEATURE: Added ability to copy validations from the underlying model.

It works like so:

```ruby

class Album
  include ActiveModel::Model
  attr_accessor :title, :artist

  validates :title, :artist, presence: true
end

class AlbumForm < Reform::Form
  extend ActiveModel::ModelValidations

  property :title
  property :artist_name, as: :artist

  copy_validations_from Album
end
```

And for composite forms:

```ruby
class AlbumRating
  include ActiveModel::Model

  attr_accessor :rating

  validates :rating, numericality: { greater_than_or_equal_to: 0 }

end

class CompositeForm < Reform::Form
  include Composition
  extend ActiveModel::ModelValidations

  model :album

  property :title, on: :album
  property :artist_name, as: :artist, on: :album
  property :rating, on: :album_rating

  copy_validations_from album: Album, album_rating: AlbumRating

end
```

I don't think it works for uniqeness validations, because I think reform only intercepts `validates_uniqueness_of`, not the `validates :uniqueness` syntax, which this uses. I will add tests for & fix this.

I also think that `representable_class.representable_attrs` (or some higher-level abstraction of the raw configuration data) would benefit from being an instance of a Mapping class rather than just a hash, so that forward and reverse mappings could be done with just one method call.
In retrospect, unsure how I feel about passing a possibly-nil model_name in almost every method in this class. Seems like a code smell to me. It'd probably be better to instantiate one ValidationsCopier class per model. Want me to update this in a PR?