[data/values] support specifying data values via cmd args that are not present in file data values

in some cases it's useful to specify data values that are not present in file data values via cmd arg. since cmd args are overlayed on top of file data values they need to have annotation `overlay/match missing_ok=True` set on them.

```
ytt -f config.yml -v something+.namespace=blah
```

we can try to assign special character (`+`) that could be used to indicate that.

TBD:
- should this also apply to env vars? (env vars are constrained to `a-zA-Z0-9_` unfortunately)

👍 "I have this use-case and would be served by having this feature implemented"
👎 "I think implementing this feature would do more harm than good."

Start by tracing ytt's command-line data-value overlay handling and how the overlay/match missing_ok=True annotation is applied. Resolve the proposed special-character syntax, decide whether environment variables are in scope, and verify that a command argument can provide a value absent from the file data values.

Written by the indexing model from the issue text.