Add parseYAML helper

[fernferret]

Why

Consul-template supports parseJSON. The consul ui supports syntax highlighting for both JSON and YAML so it'd be ncie if consul-template had first-class support for both reading and writing yaml.

What

This adds a new helper parseYAML that works identically to parseJSON but accepts YAML.

YAML is much nicer to use for config files than JSON mainly because
comments can be used.

It should be noted that parseYAML | toYAML will strip comments with
the current yaml library, but I don't want to go about changing
that huge of a dependency at this stage. yaml.v3 does support comments,
but it's not super easy to use as you can't just unmarshal to a
map[string]interface{} and keep comments.

Other details

To work around this, I'd made a plugin yaml2json which was just a tiny go program but then I have to carry that around in my $PATH and I have to use the following snippet which feels a bit meh given how nicely the parseJSON parser works.

{{ with $d := key "user/info.yaml" | plugin "yaml2json" | parseJSON }}
{{ $d.name }}
{{ end }}

This fixes #1343

[hashicorp-cla]

All committers have signed the CLA.

[fernferret]

Hmm looking at the build logs, vault tried to spin up but took a little while to migrate and thus the secret push failed.

Can someone with perms try re-running the pipeline? I don't believe this change should have failed anything and I did add several tests for my new function which passed with flying colors in my test area.

[fernferret]

Ah looks like I was lucky enough to trigger the test race condition :)

Ref: #1329 (comment) and https://app.circleci.com/jobs/github/hashicorp/consul-template/278

[eikenb]

Hey @fernferret, I'm starting a review of PRs for the next release. I haven't got to this one yet but I did kick the CI run and it is now passing.

[eikenb]

LGTM