Accept input to `jk validate` on stdin

[squaremo]

It'd be handy to be able to pipe things into jk validate (et al?).

[squaremo]

This would probably need to default to YAMLStream, and take a flag which forces it to a particular format.

curl -L https://raw.github.com/some/random/kubernetes.yaml | jk validate --format yamls ./ok.js -