adding keys is not intuitive

i have a keypair `key.json` and there's seemingly no way to add it:

```
$ jup keys add cavey key.json
error: too many arguments for 'add'. Expected 1 argument but got 2.
```

```
$ jup keys edit cavey --private-key key.json
Error: Invalid private key format
```

it's also not clear whether hardware signing is supported
