Feature Request: Support Secret sharing

[yujinqiu]

Hi,

age is a great cli tool, really love it <3. Can support key sharing , like this project
[https://github.com/onetwopunch/sham], so I don't need two cli.

Why I need this feature ?
I want to sharing my key to 3 person and once more than 2 people agree to see the content.

It can be better if age can support specified ssh key.

[covert-encryption]

Duplicate of #305