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

[mikonieminen]

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:

sops exec-file --output-type json --extract '["server"]["privatekey"]' secrets.yaml "yarn start --private-key {}"

[mikonieminen]

Seem that implementing this was quite easy, so I've created PR for it: #1098