[lang] support emitting comments in resulting YAML

originally @patricknelson added as a comment to related issue: https://github.com/k14s/ytt/issues/14#issuecomment-542965903:

It's not always essential for the resulting YAML to have comments for me, but it certainly would be very helpful!

What if we were to use a double `#` to work as an escape method when at the beginning of a line, so that the resulting YAML will contain a standard comment? For example, taken from the main demo at https://get-ytt.io:

```yaml
#@ def labels():
#! This is a ytt comment
## This is a yaml comment
app: echo
org: test
#@ end

kind: Pod
apiVersion: v1
metadata:
name: test-comments
labels: #@ labels()
```

With the output resulting in:

```yaml
kind: Pod
apiVersion: v1
metadata:
name: echo-app
labels:
# This is a yaml comment
app: echo
org: test
```

I'm brand new to `ytt` (just doing research right now), so I'm asking just in case this isn't already available with first class syntax support without any special flags.