Allows ACLs/settings in model config

/to @bajtos @ritch 

The PR allows acls and other setting properties to be specified in model-config.json.
Please log a message when a property was skipped, so that users have an easy way of troubleshooting their model-config.json. I slightly prefer `console.warn`, but I am ok with `debug` too.

I was originally thinking that these extra options should be grouped in an `options` object, to keep the top-level entry neat and predictable:

```js
{
  dataSource: "db",
  relations: { /* ... */ },
  acls: [ /* ... */ ],
  options: { /* ... */ }
}
```

This is important especially for loopback-workspace & friends, as it allows workspace API to specify all top-level properties in the domain model (`ModelConfig.acls: array`, `ModelConfig.options: object`) and provide a canonical order of these top-level properties that remains always the same across load/save roundtrips.

Thoughts?