Improve unique value support

[Sawy7]

Hello, I would like to submit these changes aiming to improve unique value generation that was introduced here. The main focus of this PR is to allow accessing every property through Faker().unique and to introduce a mode in which unique values are preferred, but not enforced (Faker().preferred_unique). Full set of changes bellow.

What does this change

• Allow accessing any property through UniqueProxy
• Add preferred uniqueness (won't raise if unique values exhausted)
• Exempt generator methods starting with 'current_' from uniqueness checks and make their values initially unique with multiple locales
• Modify and add new tests

What was wrong

These are quality-of-life improvements, rather than bugfixes.

How this fixes it

Slight additions to Faker class. Most modifications are in UniqueProxy.

Checklist

• I have read the documentation about CONTRIBUTING
• I have read the documentation about Coding style
• I have run make lint

Thanks for your consideration. Please let me know if there is anything I can change to make this work for inclusion in the project.

[Sawy7]

Hey @fcurella, sorry to bother you. Just wanted to ask if there is something I could do in order to make the inclusion of my changes possible. Thanks!