Add unslug() to UniqueSlugify() to reverse map slugs

[sanand0]

Since UniqueSlugify already maintains a set of used uids, it's a small step from there to map these back to the original text used to create them.

For example, if the username John Smith is slugifyed into john-smith and used as the URL fragment example.org/user/john-smith, the handler can go from the slug john-smith back to the original name John Smith easily.

@dimka665 -- I've added test cases for this scenario. The existing tests aren't breaking either. Would you consider this merge request?