Allow to add a list of enrichers / fetch properties for a given entity name

Make it possible to add an array of enrichers / fetch properties, without overriding the existing ones.
Before:

  nuxeo.enricher('document', 'breadcrumb')
    .enricher('document', 'thumbnail');

After:

  nuxeo.enrichers({ document: ['breadcrumb', 'thumbnail'] }, false);

Using the existing enrichers() method (in its current state) overrides completely the existing enrichers.
Added a boolean parameter to specify whether we want to override, or merge the enrichers. Default to override.