Add form fields for all django forms

[timobrembeck]

Add form fields for all forms which inherit from django.forms.BaseForm (e.g. django.forms.ModelForm), not only from forms inheriting from django.forms.Form.

The method _add_form_fields() only uses the form attribute base_fields which is defined in the BaseForm-class, so the method should work for any kind of django forms, no matter their inheritance path.

Fixes #27

[codecov-commenter]

Codecov Report

Merging #28 into master will increase coverage by 4.45%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   62.67%   67.12%   +4.45%     
==========================================
  Files           6        6              
  Lines         217      219       +2     
==========================================
+ Hits          136      147      +11     
+ Misses         81       72       -9     

Impacted Files	Coverage Δ
sphinxcontrib_django/docstrings.py	53.02% <100.00%> (+6.04%)	:arrow_up:
sphinxcontrib_django/tests/test_docstrings.py	96.29% <100.00%> (+0.14%)	:arrow_up:

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[timobrembeck]

Hi @vdboor, do you still maintain this project? If not, would you accept help? I would prefer to contribute to this project than to work on my own fork.

[timobrembeck]

This was fixed in 52dce804affab2b28e6f35cc0fc2e9f66efe498d