Load configuration file if it exists

[skalee]

If config/attr_masker.rb exists, then it is loaded from a Rake task. Fixes #118.

[codecov]

Codecov Report

Merging #122 (7cbb227) into master (5bc7741) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage   99.46%   99.48%   +0.02%     
==========================================
  Files          24       26       +2     
  Lines         371      388      +17     
==========================================
+ Hits          369      386      +17     
  Misses          2        2              

Impacted Files	Coverage Δ
lib/tasks/db.rake	90.90% <100.00%> (+2.02%)	⬆️
spec/dummy/config/attr_masker.rb	100.00% <100.00%> (ø)
spec/support/force_config_file_reload.rb	100.00% <100.00%> (ø)
spec/unit/rake_task_spec.rb	100.00% <100.00%> (ø)

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[skalee]

Converting to draft. I forgot to update docs.