Allow to configure plugins with ansible.cfg keys and env variables

[felixfontein]

Motivation

Fixes #80.

Changes description

Allows to configure generic plugin options with ansible.cfg keys and env variables.

[codecov]

Codecov Report

Merging #81 (e4218ba) into main (9ce9e0c) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   64.37%   64.45%   +0.07%     
==========================================
  Files           9        9              
  Lines         946      948       +2     
  Branches      217      217              
==========================================
+ Hits          609      611       +2     
  Misses        255      255              
  Partials       82       82              

Impacted Files	Coverage Δ
plugins/lookup/sops.py	100.00% <ø> (ø)
plugins/vars/sops.py	89.83% <ø> (ø)
plugins/doc_fragments/sops.py	100.00% <100.00%> (ø)

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[felixfontein]

@Lycowolf @briantist thanks for reviewing and testing!