Ability extract sub-part of the document tree with exec-file and exec-env

While `decrypt` allows me to extract sub-part of the document tree, I cannot do this with `exec-file` or `exec-env`. 

Having ability to do this would be very useful.

I would expect something like the following to work:
```sh
sops exec-file --output-type json --extract '["server"]["privatekey"]' secrets.yaml "yarn start --private-key {}"
```