Add ability to map additional relational links via metadata map

Oftentimes it is useful to include additional links for resources. As an example:

```json
{
    "_links": {
        "self": {"href": "..."},
        "phones": {"href": ".../phones"}
    },
    /* ... */
}
```

Potential issues in implementation:

- Such links often are derived from the current resource; how can we get the identifier into the link?
- Would an event system make more sense? or any sense?