When opening external links in eww, blink the link

When opening external links in eww, blink the link

* lisp/net/eww.el (eww-follow-link): Ditto.

* lisp/net/shr.el (shr-selected-link): New face (bug#25096).
(shr--blink-link): New function to blink links.
(shr--current-link-region): New utility function.
(shr-browse-url): Use it to blink external links.

Blinking the link allows the user to get immediate feedback that the
action has been performed.  Opening the external browser may take a
while, and may not be obvious that is going on.