RFE: Adding array items with from the command line

I have a file `values.yml` that looks like this:

```
#@data/values
---
namespace: default
flavor: medium
disk: 10Gi
ssh_keys: []
```

I would like someone running `ytt` to be able to easily append values to the `ssh_keys` array from the command line (without having to write a YAML list inline). I was envisioning something like this:

```
ytt -f template.yml \
--data-value-file ssh_keys+=$HOME/.ssh/id_rsa_foo.pub \
--data-value-file ssh_keys+=$HOME/.ssh/id_rsa_bar.pub
```

That is, use `+=` to indicate "append to list", rather than trying to set a key on a mapping. Does that seem reasonable?

Start at the ytt command-line handling for --data-value-file, using the values.yml example and template.yml invocation as the behavioral reference. Check how repeated options and ssh_keys: [] are currently processed; done means += appends each supplied file's value to ssh_keys without requiring an inline YAML list.

Written by the indexing model from the issue text.