The registry config.json contains an extra field, "commands",
an object. The fields of this object each is a command, which
records which versions (if any) of this command that registry
supports. A complete registry, as of today, would have these
commands:

```json
{
    "login": ["v1"],
    "publish": ["v1"],
    "owner": ["v1"],
    "search": ["v1"],
    "yank": ["v1"]
}
```

Prior to this commit, the login flow for registries other than the
default does not support the token pasting flow. Now, if the host
flag is specified, or the registry claims to support the "v1" flow,
we enter the token pasting flow.