feat($dragImage): service for simplifying dragImage generation

Added the `$dragImage` service, which allows much simpler generation of the `dragImage` object. If the interpolated value of the `drag-image` attribute  (introduced in #23) is a string, it is now converted to an image automatically. The generated image can be completely customized by overriding `$dragImageConfig` properties.
Perhaps it makes sense to be able to override the default options when calling `$dragImage.generate()`?