Commissioning support in keytool-cli for did:key/secp keys?

Blockchain Commons has an open-source project keytool-cli  (
https://github.com/BlockchainCommons/keytool-cli) written in C/C++ that
given a random seed (for instance,
https://github.com/BlockchainCommons/seedtool-cli) and an HD path will
derive a variety of hd-keys, base58 addresses, etc. (full list of output
formats:
https://github.com/BlockchainCommons/keytool-cli/blob/master/MANUAL.md#formats
).

For instance:

```
keytool \
  --seed 1eb5338edac6fae54cbb172091ae6c1a \
  master-key \
  master-key-base58
```

We would like to flat-rate commission a PR to this project to offer as
output the public key formats used by did:key for secp-based keys, along
with tests, under our open software developer contract
https://github.com/BlockchainCommons/Open-Development/tree/master/open-software-developer-contract,
with the final project licensed BSD-2-Clause Plus Patent License.

If someone is interested in tackling this relatively simple and well-scoped
project, let me know.

-- Christopher Allen

P.S. Future work could include adding schnorr signature support for did:key
(a much harder project in C/C++). Currently keytool-cli can only sign
bitcoin PSBTs.