Need support for periods in an option string?

let say I want options like `--vpn.user=bob`; `vpn.host=sub.domain.net`; ...
adding

```
vorpal
.command('...')
.option('--vpn.user', '...')
.option('--vpn.host', '...')
```

would throw an error as `options.vpn` is not set

so we need to do

```
vorpal
.command('...')
.option('--vpn')
.option('--vpn.user', '...')
.option('--vpn.host', '...')
```

even we don't have usage for vpn option.
this pollute the help generation, is prone to making errors and is kind of a boiler plate

maybe we could use something like https://www.npmjs.com/package/objectkit to auto create path ?