Added get_dialect func to have ability to pass json_serializer

[Gr1N]

Hi,

I would like to introduce simple change. In my cases I want to use another JSON serializer, for now I need to copy whole dialect initialization code into my sources. I don't want to do this. So in PR you can find new function called get_dialect. Changes backward compatible, but I'm open to make any changes and improvements.

Best regards

[codecov]

Codecov Report

Merging #451 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
+ Coverage   93.57%   93.57%   +<.01%     
==========================================
  Files          25       25              
  Lines        3672     3675       +3     
  Branches      194      194              
==========================================
+ Hits         3436     3439       +3     
  Misses        198      198              
  Partials       38       38

Impacted Files	Coverage Δ
aiopg/sa/engine.py	100% <100%> (ø)	⬆️

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[Gr1N]

@vir-mir could you please release new version of package?

[vir-mir]

@Gr1N no, I do not release releases

@asvetlov when the new release?