Add OpenArena.app v0.8.8

> OpenArena is a community-produced deathmatch FPS based on GPL idTech3 technology.

> There are many game types supported including Free For All, Capture The Flag, Domination, Overload, Harvester, and more. 

What more can I say? This game = awesome :bomb: :gun: 

The game requires a `chmod +x` to work. I added that as an `caveat`, but not sure there's a better way around it.

Thanks!