[Feature] Unregister options

[thomascgray]

Description

A new config object is given to unregisterNode(). One of these options, forceRefocus will make LRUD find a new node to focus on if unregistering the currently focused node.

To match current behaviour, forceRefocus defaults to true.

Also fixes an underlying refocus on unregister issue.

Motivation and Context

Suggested by https://github.com/bbc/lrud/issues/23

Agree in principle - there are simply some scenarios where LRUD will not be able find a good fit to focus on. In this case, the developer should call assignFocus after unregisterNode in API-land.

How Has This Been Tested?

New unit tests

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[thomascgray]

@NickGoward how do this look to you?