Allow display "sold out" or "future" tickets

[2ndkauboy]

Some WordCamps (like WordCamp Europe) do release tickets in batches. If those WordCamp do have to types of tickets on a different price, such as"general admission" and "community sponsors", some attendees might just buy the higher priced ticket, because they are not aware that a "regular price" ticket might be available soon.

If we add an option to show "sold out" tickets, any attendee would see, that such a ticket exists.

With the possibility to show "future" tickets, we can add the tickets for Contributor Days very early on and let attendees see, when these tickets would become available. We could also use this to show the next batch of regular tickets (and when they are going to be available).

Currently, any ticket that is no longer available (or not yet) is hidden from the shortcode output.