Add mechanism for `JacksonJrExtension`s to access state of `JSON.Feature`s

(note: needed f.ex for #51)

Currently Jackson jr extensions do not have means to access state (enabled/disabled) of `JSON.Feature`s. There should be something like:

```
public class ExtensionContext {
   public boolean isEnabled(JSON.Feature feature);
}
```

so that extensions can check state of Features during registration.
