Add support for custom Paperclip mappings

Paperclip supports custom interpolations in images' paths. In order to
ease transition into CarrierWave, support the same functionality with the
`Paperclip::Compatibility` module.

The public api is similar to Paperclip's

```ruby
class CustomUploader < CarrierWave::Uploader::Base
  include CarrierWave::Compatibility::Paperclip

  interpolate :some_method do |custom, style|
    custom.model.some_method
  end
end
```

This is accomplished by appending the desired functionality into
`CarrierWave::Compatibility::Paperclip#mappings`