Add Select2 mixin that uses Django's own select template

[codingjoe]

No description provided.

[codecov]

Codecov Report

Merging #116 (1ab73e5) into master (b858953) will decrease coverage by 0.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
- Coverage   99.59%   99.21%   -0.39%     
==========================================
  Files           7        7              
  Lines         246      254       +8     
==========================================
+ Hits          245      252       +7     
- Misses          1        2       +1     

Impacted Files	Coverage Δ
django_select2/__init__.py	100.00% <100.00%> (ø)
django_select2/forms.py	99.46% <100.00%> (-0.54%)	⬇️

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