allow export account without domains

[MaZderMind]

This PR allows creating an account which can then be used to create multiple clients for multiple domain requests handled in parallel.

Example:

    import simple_acme_dns
    account_client = simple_acme_dns.ACMEClient(
        directory="https://acme-staging-v02.api.letsencrypt.org/directory" if USE_LE_STAGING_ENV else "https://acme-v02.api.letsencrypt.org/directory",
        new_account=True,
        generate_csr=False
    )
    account = account_client.export_account(save_certificate=False, save_private_key=False)

…

    client = simple_acme_dns.ACMEClient.load_account(account)
    client.domains = [domain, f'*.{domain}']
    client.nameservers = ["8.8.8.8", "8.8.4.4"]
    client.generate_private_key_and_csr(key_type='ec384')
    client.request_verification_tokens()

[jaredhendrickson13]

LGTM, thanks!