Add extension for circe

[alonsodomin]

No description provided.

[codecov-io]

Codecov Report

Merging #127 into master will decrease coverage by 1.13%.
The diff coverage is 80.26%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
- Coverage   92.62%   91.49%   -1.14%     
==========================================
  Files          66       67       +1     
  Lines        1099     1140      +41     
  Branches       51       61      +10     
==========================================
+ Hits         1018     1043      +25     
- Misses         81       97      +16

Impacted Files	Coverage Δ
core/shared/src/main/scala/cron4s/package.scala	0% <ø> (ø)	⬆️
...hared/src/main/scala/cron4s/datetime/package.scala	0% <ø> (ø)	⬆️
...e/shared/src/main/scala/cron4s/expr/wrappers.scala	61.7% <ø> (+4.25%)	⬆️
...red/src/main/scala/cron4s/validation/package.scala	80% <ø> (-20%)	⬇️
...re/shared/src/main/scala/cron4s/Interpolator.scala	66.66% <ø> (ø)	⬆️
...ecline/src/main/scala/cron4s/decline/package.scala	100% <ø> (ø)	⬆️
core/shared/src/main/scala/cron4s/Cron.scala	100% <ø> (ø)	⬆️
core/shared/src/main/scala/cron4s/expr/parts.scala	33.33% <0%> (-33.34%)	⬇️
core/shared/src/main/scala/cron4s/parser.scala	100% <100%> (ø)
...main/scala/cron4s/testkit/gen/CronGenerators.scala	100% <100%> (ø)	⬆️
... and 8 more

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