Use pinentry if available

[y0ast]

rage has pinentry support: str4d/rage#64

It would be great if age could have it too.

From yubikey-agent it seems that it can be done using: github.com/gopasspw/gopass/pkg/pinentry

https://github.com/FiloSottile/yubikey-agent/blob/4998d8c3d76cbfa97fd1af2953d8eea36ae290e8/main.go

[FiloSottile]

I've decided we won't provide alternative password input methods in cmd/age, but we'll ship them all in a separate plugin, like sshpass. I'll make sure to support pinentry there, and I'll update discussion #256 when it's ready. See #256 (comment) for a bit more details.