widget support the kwargs to add custom html attributes

[thivolle-cazat-cedric]

it possible to add custom html attributs to the recaptcha widget, to add id or class.
Default values is the field.id for id and 'g-recaptcha' for the class.

[codecov-io]

Codecov Report

Merging #353 into master will decrease coverage by 0.1%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #353      +/-   ##
==========================================
- Coverage   99.67%   99.56%   -0.11%     
==========================================
  Files          18       18              
  Lines         911      917       +6     
  Branches       74       77       +3     
==========================================
+ Hits          908      913       +5     
  Misses          3        3              
- Partials        0        1       +1

Impacted Files	Coverage Δ
flask_wtf/recaptcha/widgets.py	96.77% <88.88%> (-3.23%)	⬇️

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