Added a border to links when they are clicked

One feature of RES that I like is that when you click a link, the title of the link on reddit is given a subtle border. Often I find myself opening a bunch of links in tabs, and at some point I'll want to go back to HN and read the comments, but it's difficult to distinguish which links I just clicked through the sea of other greyed out links I've viewed in the past. This allows you to quickly see which links you've clicked this 'session' distinct from other visited links.