UX: list-keys option and less manual key(files) management

[alxiong]

1. similar to gpg --list-keys, would be nice if there's a canonical path to all age's keys (e.g. ~/.config/age/keys.txt) and default delimiter would be added between keys when invoking age-keygen so that a simple parsing and regex matching could support list-key command

2. in general, key(file) management still involves a bit of manual work, which could be deliberately by design (idk) for simplicity or maybe other reasons, but as a user, it will be great to not care about details like umask in warning message is confusing #75 .
   (I guess I was used to the ssh-keygen way)

[FiloSottile]

The absence of a keyring is intentional. Keys are cheap to make, so it's expected users will make new key pairs for each application/use, and explicitly refer to them as files. Avoiding GPG's implicit keyring semantics was an original goal of the design.

[alxiong]

I see, understood. Your reply here #104 (comment) is pretty clear.
Thanks!