Add a flag to create file from output

Imagine following use case:

```sh
helm template my-app ./umbrella-chart | kbld -f - -f umbrella-chart/kbld-sources.yaml --lock-output .umbrella-state/kbld.lock.yml --registry-verify-certs=false > ./.umbrella-state/state.yaml
```

An error by `kbld` will clear the file `./.umbrella-state/state.yaml`. It's not possible to handle this case in an OS-independent construct. Providing a flag to create a file would be handy.

```sh
kbld -f - -f umbrella-chart/kbld-sources.yaml -o .umbrella-state/state.yaml
```