add history change reason to allow reasoning for the changes[2]

[joaojunior]

This pull request replace the PR #152.
I send a message to @hmit, author of the PR #152 to ask about the modifications requested by @macro1. Then, I create this new PR enjoying the code from @hmit and make the changes requested by @macro1.

I added a function that you can call explicitly and pass the change reason. This function will be call, after a save or delete operations in the original model.

[codecov-io]

Codecov Report

Merging #275 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
+ Coverage   97.48%   97.54%   +0.05%     
==========================================
  Files          12       13       +1     
  Lines         557      570      +13     
  Branches       66       69       +3     
==========================================
+ Hits          543      556      +13     
  Misses          7        7              
  Partials        7        7

Impacted Files	Coverage Δ
...ests/migration_test_app/migrations/0001_initial.py	100% <ø> (ø)	⬆️
simple_history/registry_tests/tests.py	100% <100%> (ø)	⬆️
simple_history/models.py	96.68% <100%> (ø)	⬆️
simple_history/utils.py	100% <100%> (ø)

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