Feature/a11y focus

[hirasso]

Closes #42

Description

Allows to customize the focus for a visit per fragment rule, if Accessibility Plugin is installed.

This will not move the focus at all:

{
  focus: false
}

Exception: If the currently focused element will be replaced during the visit, the focus will automatically be moved to the body by the browser.

This will move the focus to the first element matching #my-element:

{
  focus: '#my-element'
}

Checks

• [x] The PR is submitted to the master branch
• [x] The code was linted before pushing (npm run lint)
• [x] The documentation was updated as required

[daun]

Looking great! 🍾 Just found a tiny typo, added a comment.