add ability to read input file from stdin

[jpiper]

What type of PR is this?
/kind feature

What this PR does / why we need it:

This allows the tool to read input from stdin by using the special filename - (like kubectl does)

Does this PR introduce a user-facing change?:

added ability to read input-file from stdin

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jpiper
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[k8s-ci-robot]

Hi @jpiper. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[rikatz]

/ok-to-test

[rikatz]

/lgtm
/cc @LiorLieberman

Thanks!

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[rikatz]

not really a big deal, but probably you could simply check if the len of stdIn is > 0 :)

but lgtm

[rikatz]

@LiorLieberman leaving the final review for you :)