New widgetLoader and 'config' directory.

1. `widgetLoader` function in `Controller.js` re-written to allow for separate (optional) config files for widgets.

2. moved `viewer/config.js` to `config/viewer.js`

3. The options for

    * basemaps
    * bookmarks
    * identify
    * find

widgets now located in individual files in config directory. i.e., `config/basemaps.js`