Configurable by directory

What about configuration for which items in the `urlMap` are _busted_?

For example this way `/skin` and `/js` can be in the local filesystem with Apache or Nginx doing the rewrite magic but `/media` can sit in a S3 bucket or similar - with potentially nothing in-front remapping the file served. 

I believe that for most use cases the media directory has pretty good behaviour by default.

https://github.com/gknoppe-guidance/magento-cachebuster/blob/master/src/app/code/community/Guidance/Cachebuster/Helper/Data.php#L54