Support custom elements for the root of widgets

[jkotests]

Registering a widget assumes that there there is a related Watir method for your desired element type. This works for standard element types, but fails custom element types - eg Angular.

This issue has come up at least 2 times:

• https://stackoverflow.com/q/65852406/1200545
• custom elements (widgets) with user defined tag names #451 (comment)

The workaround was to create associated methods in Watir. I think the better solution is to be able to define required identifiers/locators (ie tag_name) for the widget.