Improve form object attributes model referencing

[pyromaniac]

Now it is possible to reference arbitrary attributes in model_map:

class Post::Update::ModelMap
  MAPPING = {
    %w[published_at] => Post, # a model can be passed but beware of circular dependencies, better use strings
    %w[title] => "Post", # or a model name - safer option
    %w[content] => "Post#body" # referencing different attribute is possible, useful for naming migration or translations
  }.freeze

  def call(path)
    MAPPING[path] # returns the mapping for a single path
  end
end