If no options passed from the template, a widget should default to options provided in its index.js

If we decide not to do this, we should be more explicit about it in the documentation.

Edit: use case https://stackoverflow.com/questions/47099783/options-for-areas-in-apostrophe-apos-widgets/47105098#47105098

Start by examining how options are passed from templates to a widget and how defaults are defined in the widget's index.js. Review the linked Stack Overflow use case and existing documentation, then determine whether defaulting should be implemented or documented explicitly; done means the chosen behavior is clear and covered by the relevant documentation or tests.

Written by the indexing model from the issue text.