Support merging generic related fields

[mumumumu]

Had to dig into the Django generic foreign relation internals to figure out how to pull out the name of the related generic foreign key field.

Closes #5
Closes #6

[codecov]

Codecov Report

Merging #32 into master will increase coverage by 2.84%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   96.26%   99.10%   +2.84%     
==========================================
  Files           7        1       -6     
  Lines         375      112     -263     
  Branches       24       19       -5     
==========================================
- Hits          361      111     -250     
+ Misses         13        0      -13     
  Partials        1        1              

Impacted Files	Coverage Δ
django_super_deduper/merge.py	99.10% <100.00%> (+0.05%)	⬆️
setup.py
tests/factories.py
django_super_deduper/models.py

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