Don't use color in output if piping is used

[toredash]

Suggest code change that will enable use-cases like this:
for env in $(kubectx | grep staging); do kubectx $env && kubectl scale deployments nginx --replicas=10; done

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[toredash]

I signed it!

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[ahmetb]

We got this exact same PR before. Unfortunately we don't want scripting around kubectx.

You should use kubectl config get-contexts -o name to list contexts in your script.