New Rule: UnnecessaryNotNullOperator

[cortinico]

As suggested in #2572, I'm adding a rule to detect unnecessary usages of the not-null (!!) operator.

[codecov-io]

Codecov Report

Merging #2578 into master will increase coverage by 0.04%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master    #2578      +/-   ##
============================================
+ Coverage     25.17%   25.21%   +0.04%     
- Complexity      393      394       +1     
============================================
  Files           378      379       +1     
  Lines          7399     7411      +12     
  Branches       1222     1224       +2     
============================================
+ Hits           1863     1869       +6     
- Misses         5405     5411       +6     
  Partials        131      131              

Impacted Files	Coverage Δ	Complexity Δ
...ch/detekt/rules/bugs/UnnecessaryNotNullOperator.kt	45.45% <45.45%> (ø)	1.00 <1.00> (?)
...sch/detekt/rules/providers/PotentialBugProvider.kt	100.00% <100.00%> (ø)	2.00 <0.00> (ø)

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[cortinico]

My bad I forgot to rebuild the docs