feat(clients): implement the "clients" command

This command will return a list of clients registered with the server.
The command can be passed a --token option to skip fetching a new token.
I fear that this command-line-parsing lib doesn't like having both a `--token` option and a `token` sub-command.

When I try to run it like this to generate an oauth token:

```
./bin/fxa-oauth.js --env=latest --user=rfkelly@mozilla.com token 24bdbfa45cd300c5 oauth
```

It fails out complaining about a "missing email".  Tracing it through, I can see that it's taking this `if (program.token)` branch even though I haven't specified a `--token` command-line option.  And logging the contents of `program.token` show that it's some sort of object with the command-line args etc in it.