Fix #148 add a compare fallback

[jedie]

work-a-round for: etianen/django-reversion#859

The test project creates a example. This compare must use the fallback:

The fallback compare diff looks in this case like this:

[codecov]

Codecov Report

Merging #149 (cb63680) into master (c03fed8) will decrease coverage by 0.50%.
The diff coverage is 75.91%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
- Coverage   88.02%   87.52%   -0.51%     
==========================================
  Files          33       38       +5     
  Lines        1604     1732     +128     
  Branches      164      165       +1     
==========================================
+ Hits         1412     1516     +104     
- Misses        144      168      +24     
  Partials       48       48              

Impacted Files	Coverage Δ
reversion_compare_tests/settings.py	100.00% <ø> (ø)
reversion_compare_tests/test_basics.py	100.00% <ø> (ø)
...compare_tests/migrations/0003_migration_model_2.py	27.27% <27.27%> (ø)
...compare_tests/migrations/0002_migration_model_1.py	36.84% <36.84%> (ø)
reversion_compare/compare_raw.py	88.23% <88.23%> (ø)
reversion_compare_tests/utils/fixtures.py	79.90% <88.23%> (+0.71%)	⬆️
reversion_compare/admin.py	87.27% <95.45%> (+1.71%)	⬆️
reversion_compare_tests/admin.py	100.00% <100.00%> (ø)
reversion_compare_tests/migrations/0001_initial.py	100.00% <100.00%> (ø)
reversion_compare_tests/models.py	100.00% <100.00%> (ø)
... and 8 more

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