Run tagged scenarios

Only supports "-t @tag1,@tag2", "-t ~@tag1", "-t @tag1 -t @tag2" scenarios currently.

I'm also not 100% convinced this is the best way of doing this (e.g. converting App::pherkin to a real object) but I couldn't see an easy sensible way to persist things between ->_process_arguments() and ->run() (but I've been out of Perl for a while and consequently rusty.)

Suggestions gratefully received.