Add support for alternative captcha services

[Dosenpfand]

Hi,
This merge request would add support for more CAPTCHA services, e.g. hCaptcha, a privacy focused non-Google alternative to reCAPTCHA.
If desired I could also add an additional hCaptcha entry to the examples/.

Cheers!

[codecov-io]

Codecov Report

Merging #425 (d0d897a) into master (3ed9be4) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #425      +/-   ##
==========================================
+ Coverage   97.97%   98.09%   +0.12%     
==========================================
  Files          18       18              
  Lines         936      998      +62     
  Branches       73       78       +5     
==========================================
+ Hits          917      979      +62     
  Misses         17       17              
  Partials        2        2              

Impacted Files	Coverage Δ
flask_wtf/recaptcha/validators.py	100.00% <100.00%> (ø)
flask_wtf/recaptcha/widgets.py	100.00% <100.00%> (ø)
tests/test_recaptcha.py	100.00% <100.00%> (ø)
flask_wtf/file.py	93.87% <0.00%> (+2.21%)	⬆️
tests/test_file.py	85.18% <0.00%> (+4.23%)	⬆️

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[azmeuk]

Thank you for your contribution!