Bettercreate

Currently, `gandi domain create` requires the `--domain` flag, e.g. `$ gandi domain create --domain example.com`.
I have replaced `click.option` with `click.argument` so that a domain name can be provided as a positional argument, e.g. `gandi domain create example.com` (the user will be prompted if no domain is provided).

Also, the CLI will now display the contacts of the domain being registered, and prompt the user for confirmation before attempting to register the domain:

```
$ gandi domain create exampleawefafdsf.xyz
The domain exampleawefafdsf.xyz is available. If you continue, registration will be attempted with the following parameters:

        Duration: 1 year(s)
        Owner:    XX0000-GANDI
        Admin:    XX0000-GANDI
        Tech:     XX0000-GANDI
        Billing:  XX0000-GANDI

Your account will be charged. Do you want to continue? [y/N]:
```

If this is merged, we should probably add a `--yes` option so the prompt can be overridden.