Disable uploading the krew plugin manifest using goreleaser (stern#175)

* Disable uploading the krew plugin manifest using goreleaser

* Fix indentation

Use Go 1.17 (stern#157)

No longer need to set GO111MODULE=on due to default (stern#151)

Make cmd testable (stern#132)

* Make cmd testable

* Use genericclioptions.IOStreams for stdio

* Rename completion.go to flag_completion.go

* Extract code related to --version flag to flag_version.go

* Extract code related to --prompt flag to flag_prompt.go

* Functionize code to register completion function to cobra.Command

* Add tests for cmd.NewSternCmd

* Refactor flag_version.go

* Add a test for options.Validate method

Generate cli flags section on README.md (stern#131)

* Reorder opts

* Set opts.since default value

* Move flags into AddFlags function to use hack/update-readme

* Implement hack/update-readme

* Implement hack/verify-readme

* Update README.md which is generated by update-readme

Suuport ephemeral container logs (stern#125)

Add --exclude-pod flag (stern#122)

Update Kubernetes vendors to v1.21.0 (stern#119)

Add field selector (stern#111)

Fix mixed log lines (stern#106)

> A template may be executed safely in parallel, although if parallel
executions share a Writer the output may be interleaved.

For the above reason, the result of the template's execution must be
stored in a buffer once.