Find Hints Within Shadow Roots

Vimium currently cannot identify buttons, links, etc. when they are buried within the Shadow DOM (example). A minor modification to make the look-up method recursive could resolve this. See the following for a reduction of the issue: https://jsfiddle.net/gh5m94p7/4/.

And example look-up approach

// Pass in the root element, and whether or not to pierce Shadow DOMs
const getAll = ( root, includeShadows ) => {
  // Document Fragments support querySelectorAll
  let items = Array.from( root.querySelectorAll( '*' ) )
  items.forEach( item => {
  	if ( item.shadowRoot )
        // Recursively call getAll to include shadow roots
    	items = items.concat( getAll( item.shadowRoot, true ) )
  })
  return items
}

Current lookup takes place here: https://github.com/philc/vimium/blob/881a6fdc3644f55fc02ad56454203f654cc76618/content_scripts/link_hints.coffee#L748

Start at content_scripts/link_hints.coffee around line 748, where the current element lookup is implemented, and compare it with the reduced Shadow DOM example in the issue. Verify the lookup across nested shadow roots and confirm that buttons and links inside them become available to link hints.