Add tests showing how to exclude custom config properties in plugins

[arturbosch]

Closes #2169

[codecov-io]

Codecov Report

Merging #2192 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #2192      +/-   ##
============================================
+ Coverage     81.12%   81.15%   +0.03%     
- Complexity     2055     2057       +2     
============================================
  Files           339      339              
  Lines          5874     5891      +17     
  Branches       1070     1073       +3     
============================================
+ Hits           4765     4781      +16     
+ Misses          536      535       -1     
- Partials        573      575       +2

Impacted Files	Coverage Δ	Complexity Δ
.../kotlin/io/gitlab/arturbosch/detekt/cli/CliArgs.kt	100% <100%> (ø)	6 <0> (ø)	⬇️
...in/io/gitlab/arturbosch/detekt/cli/BuildFailure.kt	90.62% <0%> (-4.83%)	1% <0%> (ø)
...n/io/gitlab/arturbosch/detekt/core/DetektFacade.kt	48.78% <0%> (ø)	3% <0%> (ø)	⬇️
...ekt/generator/printer/rulesetpage/ConfigPrinter.kt	75% <0%> (ø)	8% <0%> (ø)	⬇️
...osch/detekt/cli/console/FileBasedFindingsReport.kt	92.59% <0%> (+0.92%)	8% <0%> (+1%)	⬆️
...ab/arturbosch/detekt/cli/console/FindingsReport.kt	94.11% <0%> (+1.26%)	7% <0%> (+1%)	⬆️
...otlin/io/gitlab/arturbosch/detekt/api/CodeSmell.kt	60.46% <0%> (+2.32%)	11% <0%> (ø)	⬇️

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