Add target blank option to auto link

Although `:target => "_blank"` is an available option I'm proposing adding `:target_blank => true` to keep in line with the js version of twitter text option of `targetBlank`. This will make it easier for developers to move back and forth between the two libraries without having to look up documentation. This new option adds `target="_blank"` to all auto linked entities not just url entities (as in `:url_target`).