Add UniqueSlugify for generating unique slugs

[sloria]

No description provided.

[coveralls]

Coverage increased (+0.9%) to 92.982% when pulling c7c8526 on sloria:unique-slugify into 98c83be on un33k:master.

[un33k]

Unfortunately the uniqueness cannot be enforced through restart of applications or on multiple instances of the same application talking to the same backend.

As such it is best to handle this on case-by-case basis within one's application.

If you need something this for Django, you can refer to https://github.com/un33k/django-uuslug

[sloria]

There are use cases when you don't global uniqueness and per-process uniqueness is desired. For example: generating slugs for headers within a user-submitted document such as a README. In my case, users build a form by providing form field types and human-readable labels ("Email address") from which machine-readable IDs ("email-address-1") must be generated. The IDs should be unique within a form but not globally unique.

[un33k]

Only good for single instance applications. Out of sope for this module. Thx

[sloria]

The use cases I mentioned aren't specific to single-instance apps. I respect if you think this is out of scope for this library, though. Thanks anyway.

[un33k]

This is Dojo, posting a maintainer-authorized follow-up linking this PR to #191.

A stateful uniqueness API remains deferred. Collision handling belongs to the application; no database uniqueness guarantee is provided.

The default algorithm remains legacy; improved output rules are opt-in. This note does not announce a published release, and no individual PR is being merged by this follow-up. Thank you for the contribution and discussion.

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