Client commands accept a new `--keyring-backend` option through which users can specify which backend should be used by the new key store:
- os: use OS default credentials storage (default).
- file: use encrypted file-based store.
- test: use password-less key store (highly insecure).
Add Keybase's ExportPrivKey()/ImportPrivKey() API calls to export/import
ASCII-armored private keys. Relevant keys subcommands are provided as well.
Closes: #2020