Editor: fix handling of abutting scan matches.

Editor: fix handling of abutting scan matches.

My example test of '#hashtag@mention' _shouldn't_ have highlighted the
mention, because the word character before the @ disqualifies it. But
I was doing the matching on a slice of the string, instead of giving
get_span the whole string and telling it where to start from.

That in turn was because I hadn't bothered to give get_span that
argument. But it's really easy to add! Now done.