Add a THUMBNAIL_NAME_PROCESSOR

By default, easy-thumbnails creates a thumbnail from an image, then appends the list of options in clear text and the output extension.
The list of options can be quite long and can generate very long filenames, e.g. : filename.jpeg.crop-smart_radius-full_blur-100.jpg
I gave the options to define one's prefered behaviour to transform the list of options to a string. One could then write a processor to turn the name to something like : filename.jpeg.aF6pw2.jpg where aF6pw2 would be a string generated from the list options.
The THUMBNAIL_NAME_PROCESSOR by default points to a processor which retains the current behaviour of easy-thumbnails.