Exit process with specific code when detekt finds code issues

[3flex]

Closes #1493

detekt will exit with one of the following exit codes:

Exit code	Description
0	detekt ran normally and maxIssues or failThreshold count was not reached in BuildFailureReport.
1	An unexpected error occurred
2	maxIssues or failThreshold count was reached in BuildFailureReport.

[vanniktech]

No unit tets?

[codecov-io]

Codecov Report

Merging #1508 into master will decrease coverage by 0.08%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #1508      +/-   ##
============================================
- Coverage     74.81%   74.72%   -0.09%     
  Complexity     1737     1737              
============================================
  Files           328      328              
  Lines          5280     5286       +6     
  Branches        961      961              
============================================
  Hits           3950     3950              
- Misses          805      811       +6     
  Partials        525      525

Impacted Files	Coverage Δ	Complexity Δ
...ain/kotlin/io/gitlab/arturbosch/detekt/cli/Main.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
...tlin/io/gitlab/arturbosch/detekt/cli/JCommander.kt	0% <0%> (ø)	0 <0> (ø)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4a08c7c...5d86f0b. Read the comment docs.

[schalkms]

I think the error description should also be added to the documentation.

[3flex]

I looked into testing but it's a little tricky: https://stackoverflow.com/questions/309396/java-how-to-test-methods-that-call-system-exit

I'll be working on #1435 next and integration tests will cover the exit codes 1 & 2.

Will update comments and docs according to your suggestions, thanks!