Properly support multi-document yaml files.

[mattmoor]

Fixes: #116

[mattmoor]

@philwo Is CI not set up for this repo?

[mattmoor]

@pcj FYI

[philwo]

Post- and presubmit checks should now be working for this repo! Please let me know if you encounter any issues.

[mattmoor]

@philwo Updating the commit doesn't seem to have made this register with buildkite?

[buchgr]

@mattmoor fixed. should work now

[mattmoor]

@buchgr Why doesn't this show up the same way it does in `rules_docker?

[buchgr]

@mattmoor it will in the next build. you can test it out by just changing the commit hash in this PR (git commit --amend or so).

[mattmoor]

FYI, I'm going to ignore the CI failure (I/O failure) since macOS doesn't provide super meaningful coverage for rules_k8s and I've shared this with the Bazel team as an example of poor contributor UX.