Deep merge Attachment options into default_options

This is especially useful when a default option which takes a hash contains the `:all` key (e.g. `:convert_options`). Without deep merging it would be impossible to add convert options for another style without overriding the convert options for `:all` styles:

```ruby
# config/initializers/paperclip.rb
Paperclip::Attachment.default_options[:convert_options] = { all: "-background white" }

# app/models/person.rb
has_attached_file :avatar, convert_options: { thumb: "-thumbnailize" }
```
In the example above the convert options for `:all` are ignored, while I'd expect them to be applied since I didn't override them explicitly.