enable programmatic use of scalastyle

[vitaliis]

right now it is not possible to integrate scalastyle in anything as library

our current problem is integrating scalastyle into Mill build tool

this change would enable us to call scalastyle programmatically and avoid problems with System.exit

[codecov-io]

Codecov Report

Merging #331 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #331   +/-   ##
=====================================
  Coverage       0%     0%           
=====================================
  Files          62     62           
  Lines        1495   1495           
  Branches      146    146           
=====================================
  Misses       1495   1495

Impacted Files	Coverage Δ
src/main/scala/org/scalastyle/Main.scala	0% <ø> (ø)	⬆️

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