Add -c create or replace option

[feral-dot-io]

Currently agenix's CLI expects to be able to decrypt a file before operating further on it. If I don't have access to the identity file then this means either deleting the .age first or running agenix on the remote machine.

This PR helps by adding an explicit option that creates or replaces (if a file exists) a secret file without decrypting it first.

I realised partway through that this could also be accomplished by checking if stdin is a terminal but decided that a "don't decrypt" semantic option would be a better complement. So it's an alternative to #157.