allow DocBlueprintMixin to be configured using kwargs

[sjhewitt]

This makes it possible to create 2 Api instances with the UI for the same app

[codecov[bot]]

Codecov Report

Merging #259 (63ee52a) into master (807aaf1) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #259   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files          14       14           
  Lines         784      785    +1     
  Branches      143      143           
=======================================
+ Hits          782      783    +1     
  Partials        2        2           

Impacted Files	Coverage Δ
flask_smorest/__init__.py	100.00% <100.00%> (ø)
flask_smorest/spec/__init__.py	100.00% <100.00%> (ø)

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[lafrech]

Thank you for contributing.

Closing this because I'd rather take another approach. See #252.