Ensures that all links have `target="_blank"`.

Fairly simplistic approach, but should ensure reliable addition to anchor elements.

Wasn't sure if you'd want this as an option - my thinking was that it would be pretty unusual not to want links to open in new tab, particularly given the irregular way in which they are provided by Twitter (sometimes with, sometimes without `target="blank"`), however if this should be configurable let me know and I'll update.