Added alt-function to support RetinaJS libraries.

This pull requests implements an alt(ernative) option. This options will allow to map the hash prefix from an original file - say `logo.png` => `ab47.logo.png` - onto alternative versions of that file - say `logo@2x.png` => `ab47.logo@2x.png`.

This is useful when you are working with retina images, that require the files to named identically except for some part of the filename, for example `@2x`.

Request included passing unit tests.