Add hidden flag --force-colors to always print color codes

[corneliusweig]

Close #2021

[codecov-io]

Codecov Report

Merging #2033 into master will decrease coverage by 0.02%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master    #2033      +/-   ##
==========================================
- Coverage   55.96%   55.94%   -0.03%     
==========================================
  Files         173      173              
  Lines        7556     7563       +7     
==========================================
+ Hits         4229     4231       +2     
- Misses       2924     2928       +4     
- Partials      403      404       +1

Impacted Files	Coverage Δ
pkg/skaffold/color/formatter.go	80% <0%> (-10.91%)	⬇️
cmd/skaffold/app/cmd/cmd.go	74% <50%> (-1%)	⬇️

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[priyawadhwa]

LGTM, thanks for fixing this!