Allow custom core configuration in a nice way.

[vasilakisfil]

In order to add a core config I had to create the file components/ember-jstree.js which extends ember-jstree, override _buildConfig, call super, edit the config object and return it.. For instance,

import EmberJstree from 'ember-cli-jstree/components/ember-jstree';

export default EmberJstree.extend({
  _buildConfig() {
    let obj = this._super();
    obj['core']['expand_selected_onload'] = false;
    return obj;
  }
});

Although it doesn't hurt, I think it would be nice to have a better way to do that because everytime I update this addon I have to check if this method has changed.

[GCorbel]

I have the same issue. expand_selected_onload' is not in settings. I think adding a coreOptions` would be great.