Adapt drop to target a specific cached url

[paulprins]

would be called like.
$.fn.cacheImages.drop('http://my.diamonds/are-awesome.jpg');

New param would prefix existing. Both would be optional and if both are undefined it should wipe everything with the default prefix. The check on the new URI param would check for a length greater then 1, and if that is true it would attempt to only drop that value. Would be helpful on the back-end cleaning up images when they are no longer needed.

$.fn.cacheImages.drop( URI, storagePrefix )