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

Attention: Patch coverage is 90.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 96.25%. Comparing base (020a7f9) to head (4f1b757).
Report is 5 commits behind head on develop.

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

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.