Use event.composedPath() when available for "routeLinks" handler

[jimsimon]

The current handler for the "routeLinks" option does not support shadow DOM. Using event.composedPath will allow link intercepting to work for events bubbling from "open" shadow DOM's, and it will provide a performance boost on browsers that support it (everything except IE11/Edge) due to not having to traverse up the DOM tree.

[jimsimon]

I can open a PR for this if you'd like.

[vanwagonet]

That sounds great.