feature: add an option to output secrets in armor

[dlubawy]

This solves #269.

Summary: Age implementations support an --armor flag to output encrypted secrets in a PEM format that is human readable. This is useful for storing secrets in git as it makes diffs easier to view. This PR adds the ability to configure this option within the secrets.nix file as such:

{
  "secret1.age" = {
    publicKeys = "age1s4ks3q07t7vawcj4l8uvxvuft0paqwz8wr5f7dgt3ma9ah2nucgsfke7th";
    armor = true;
  };
}

[fzakaria]

Looks good to me; might want to augment the docs with an example;
might be good to articulate that the benefit to this is you'll store non-binary files in Git

I think a benefit is Git's packfile format handles text better than binary?
(git diff is also a little easier to use)

[DerRockWolf]

It would be nice if one could also enable armoring on a global level, or are there any downsides when doing this with all secrets? 🤔

[dlubawy]

@fzakaria, I made changes based on your suggestions. Let me know if I should change anything else, or if I should reword the doc edits.

[fzakaria]

Nice! 🎆 the test is sweet too.

Looks good to me; I'm not a maintainer though.
Let's see what @ryantm thinks.