Backport #2834 Add CommutativeMonoid[Option[A]] instance

[gagandeepkalra]

addresses #3143

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (scala_2.11@0980189). Click here to learn what that means.
The diff coverage is 100%.

@@              Coverage Diff              @@
##             scala_2.11    #3334   +/-   ##
=============================================
  Coverage              ?   93.38%           
=============================================
  Files                 ?      385           
  Lines                 ?     7166           
  Branches              ?      193           
=============================================
  Hits                  ?     6692           
  Misses                ?      474           
  Partials              ?        0

Impacted Files	Coverage Δ
core/src/main/scala/cats/instances/option.scala	100% <ø> (ø)
.../scala/cats/kernel/instances/OptionInstances.scala	100% <100%> (ø)

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