Add feature to run schema files after main schema file too

[nadege]

Adding a feature similar to peopledoc/django-north#37
making it possible to run schema files not only before the main schema but also after.
Marked previous option additional_schema_file as deprecated (not sure of my way of doing it though)

Also did a bit of gardening in the docs.

Successful PR Checklist:

• Tests
• Documentation (optionally: run spell checking)
• Had a good time contributing? yay 😃 But better test coverage would make contributing easier.

[codecov]

Codecov Report

Merging #57 into master will increase coverage by 8.58%.
The diff coverage is 84%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   56.35%   64.93%   +8.58%     
==========================================
  Files          14       14              
  Lines         598      616      +18     
  Branches       81       86       +5     
==========================================
+ Hits          337      400      +63     
+ Misses        254      208      -46     
- Partials        7        8       +1

Flag	Coverage Δ
#integration	45.29% <68%> (+32.91%)	⬆️
#unit	54.54% <16%> (-0.64%)	⬇️

Impacted Files	Coverage Δ
septentrion/cli.py	0% <0%> (ø)	⬆️
septentrion/configuration.py	80.88% <100%> (+4.31%)	⬆️
septentrion/migrate.py	58.42% <89.47%> (+58.42%)	⬆️
septentrion/files.py	90.56% <0%> (+3.77%)	⬆️
septentrion/core.py	76.05% <0%> (+4.22%)	⬆️

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[ewjoachim]

Perfect ! ✨

Thank you !

[ewjoachim]

Had a good time contributing? yay smiley But better test coverage would make contributing easier.

Yep #29