Add possibility to extend common providers

[DmytroLitvinov]

Closes #9

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.05%. Comparing base (cc55d48) to head (90f07a0).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/django_email_validators/providers.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
- Coverage   98.61%   96.05%   -2.56%     
==========================================
  Files           6        6              
  Lines          72       76       +4     
==========================================
+ Hits           71       73       +2     
- Misses          1        3       +2     

Flag	Coverage Δ
unittests	96.05% <50.00%> (-2.56%)	⬇️

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