Release v0.21.0

[robingustafsson]

Release branch for v0.21.0.

[codecov-io]

Codecov Report

Merging #641 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #641      +/-   ##
==========================================
- Coverage   64.56%   64.54%   -0.03%     
==========================================
  Files         101      101              
  Lines        8060     8060              
==========================================
- Hits         5204     5202       -2     
- Misses       2527     2529       +2     
  Partials      329      329

Impacted Files	Coverage Δ
cmd/root.go	30.76% <ø> (ø)	⬆️
core/engine.go	88.2% <0%> (-1.03%)	⬇️

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[luizbafilho]

DO IT! 😃