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.
What do we do if we want multiple configurations based on different dragsites?