Deleting properties

There should be NAPI functions for deleting named properties and indexed properties:
```C
napi_status napi_delete_property(napi_env env, napi_value object, napi_propertyname name);
napi_status napi_delete_element(napi_env env, napi_value object, uint32_t i);
```

This is not super high-priority, since only one of the top native modules I scanned (couchbase) referenced `v8::Object::Delete()`.