Create a new section in the meta-data called "hints" where users can
provide hints about a given property. The most basic use case for now
is to provide a list of values that a property can have. Each value may
have a description.

This sample JSON provides a basic example for a property called `foo.mode`
that exposes 3 values: "auto", "basic" and "advanced".

```
 "hints": [
    {
      "id": "foo.mode",
      "values": [
        {
          "value": "auto",
          "description": "Some smart description."
        },
        {
          "name": "basic"
        },
        {
          "name": "advanced"
        }
      ]
    }
]
```

This information can be read by tools (such as IDE) and offer an
auto-completion with the list of values.

Closes gh-2054