Added support for internal Valuer and Scanner values

[thenixan]

I've used your library to wrap uuid.UUID type and unfortunately it seemed not to be working with types that have their own Scanner and Valuer implementation, so I've made a quick fix for that

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (8b16952) 100.00% compared to head (cc43d63) 95.58%.

Files	Patch %	Lines
gonull.go	66.66%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main      #15      +/-   ##
===========================================
- Coverage   100.00%   95.58%   -4.42%     
===========================================
  Files            1        1              
  Lines           59       68       +9     
===========================================
+ Hits            59       65       +6     
- Misses           0        2       +2     
- Partials         0        1       +1     

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[LukaGiorgadze]

Hey @thenixan thank you for your contribution!
Good point, going to merge PR. However, one negative test case are missing and tests dropped by -4.42%, but I'll update it before the release.

Thanks!