Capability to load a different settings.py using a cli parameter

[lbiaggi]

As from title to use with different settings with pre-commit & similar cases in CI pipelines

Depends on: #641

[lbiaggi]

@neerdoc can you review this?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.25%. Comparing base (020a7f9) to head (4f1b757).
⚠️ Report is 155 commits behind head on develop.

Files with missing lines	Patch %	Lines
doorstop/cli/main.py	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #642      +/-   ##
===========================================
+ Coverage    96.15%   96.25%   +0.10%     
===========================================
  Files           41       41              
  Lines         5173     5182       +9     
  Branches      1214     1218       +4     
===========================================
+ Hits          4974     4988      +14     
+ Misses         127      121       -6     
- Partials        72       73       +1     

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