Add mechanism for JacksonJrExtensions to access state of JSON.Features

[cowtowncoder]

(note: needed f.ex for #51)

Currently Jackson jr extensions do not have means to access state (enabled/disabled) of JSON.Features. There should be something like:

public class ExtensionContext {
   public boolean isEnabled(JSON.Feature feature);
}

so that extensions can check state of Features during registration.