[Updated] formatting for nested attributes

[broodfusion]

Hi @beerlington, I submitted a new PR here.
Continuation from #295

[broodfusion]

FYI, could probably add this in the readme file:

To serialize nested attributes in a Phoenix app, include this anywhere:

defimpl JaSerializer.Formatter,
  for: [JaSerializer.Builder.Attribute] do
  alias JaSerializer.Formatter.Utils

  def format(%{value: value, key: key}) when is_map(value) or is_list(value) do
    values = Utils.deep_format_keys(value)
    {Utils.format_key(key), JaSerializer.Formatter.format(values)}
  end

  def format(attr) do
    {Utils.format_key(attr.key), JaSerializer.Formatter.format(attr.value)}
  end
end

Couldn't have this in attribute.ex because it would break the tests in attributes_test.ex for "allows overriding for struct formatting" and "map formatter can be changed".

Thanks!