Allow to generate custom suffixes

[josepdecid]

• Added the ability to customize the generated suffix to ensure uniqueness.
• Allow to force the suffix to be appended even to the first occurrence of the slug.

[freekmurze]

This is a description of the functionality, could you also add a simple example on how such a slug would look like. That would really drive the point home.

[josepdecid]

Done! Feel free to suggest any other change or improvement!

[freekmurze]

Thank you!