`Leaflet.translate` — add namespaces; support 'i18next'

.. "core" and "plugins", initially:

* https://www.i18next.com/principles/namespaces

```js
i18next.init({
  resources,
  ns: ['core', 'plugins'],
  defaultNS: 'core'
})
```
..

---
Was: https://github.com/nfreear/Leaflet.translate/issues/5.