crossover matches - pseudo double elimination

[clux]

See tournament-js/tournament#6

Dropped this in tournament earlier on, but may be worth looking at at some point.

Dynamic FFA first though: tournament-js/tournament#14

[clux]

FFA is already pretty advanced. With the latest advancements of clux/ffa-tb and restrictions on FFA.invalid you can already do things that other tournaments offer (Masters/Knockout is a specialization of FFA, single elimination Duel can be created from FFA), so adding something like this is a lot of extra complexity. Any thoughts on this is welcome, but for now this will not be prioritized.

[clux]

Maybe nice work for a subclass.