allow a Vignette url to change formats after the url has been generated

@Wikia/platform-team @kvas-damian 
https://wikia-inc.atlassian.net/browse/PLATFORM-647

`ImagesService:: overrideThumbnailFormat()` is called after the url for an image has been generated, so we need a way to change the output format from a string :cry: 
Sure, I think that reads better for sure