Don't document class attributes that are an alias to another object

[theOehrly]

Closes #69

This is WIP for now, still needs tests. And I need to check if there aren't any unfortunate side effects. I'll have a look at how Sphinx itself detects an object as an "alias" because it clearly knows. Then I'll finalize this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.95%. Comparing base (6834615) to head (73baf9e).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
- Coverage   61.89%   60.95%   -0.95%     
==========================================
  Files           2        2              
  Lines         664      671       +7     
==========================================
- Hits          411      409       -2     
- Misses        253      262       +9     

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[theOehrly]

Test added, and I switched to using a flag that ClassDocumenter already provides and uses internally to mark the attribute as alias instead of doing my own checks.

[Chilipp]

awesome! thanks @theOehrly