age-keygen: Add option to compute pubkey from privkey

[dsprenkels]

This PR adds an option to age-keygen to compute an Curve25519 public key from the respective private key.

It currently implements this:

Usage of ./age-keygen:
  -o FILE
        output to FILE (default stdout)
  -pubkey
        Read the private key from standard input print the corresponding public key.

This is different from what I proposed in issue #122, in the way that it only reads from standard input. --pubkey does not take an (optional) file argument.

The reason is that in order to implement this, we need to overhaul the command line flag parsing, including the usage string. It feels like that is a bit too involved for such a small change.

In the future, we can update age-keygen such that it also accepts -p (as an alias to --pubkey) and that the option can take an optional file argument, from which the private key should be read.

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Fixes #122

[FiloSottile]

Thank you for the PR! I went with something a little different, stealing pieces of ssh-keygen and cmd/age, but I liked the idea of using standard input by default!