Add unimplemented 'skaffold render' command

[nkubala]

Relates to #1187

This adds a new command, skaffold render, which will run builds for all artifacts, tag the images, and then output the templated kubernetes manifests to the user rather than actually running the deploy. The manifests are written to stdout by default, but a --output flag can be passed to provide a filename for them to be written to. These manifests can be manually applied by the user if desired, or committed to a repository for use in CI/CD or GitOps workflows.

This does not add implementations for any deployers. Those will come in follow up PRs.

This also adds a flag, --render-only to skaffold dev and skaffold run, which will exhibit the same behavior as skaffold render (though in dev mode it will continuously print the templated manifests after each successful build).

By default, the logs and output for the builds are suppressed, but can be displayed using a --loud flag.

[codecov]

Codecov Report

Merging #2942 into master will decrease coverage by 0.08%.
The diff coverage is 37.5%.

Impacted Files	Coverage Δ
pkg/skaffold/runner/runner.go	0% <ø> (ø)	⬆️
pkg/skaffold/config/options.go	100% <ø> (ø)	⬆️
cmd/skaffold/app/cmd/flags.go	100% <ø> (ø)	⬆️
pkg/skaffold/runner/render.go	0% <0%> (ø)
pkg/skaffold/runner/deploy.go	66.66% <0%> (-7.02%)	⬇️
cmd/skaffold/app/cmd/cmd.go	68.25% <100%> (+0.25%)	⬆️
cmd/skaffold/app/cmd/render.go	42.1% <42.1%> (ø)