The annoying thing about wither skeletons is: There is no way I know of to spawn them in the overworld!

I have determined that there is no way to spawn wither skeletons in the overworld without commands. So that is what I am looking into. 

I want to know if there is a command that summons wither skeletons so then I can play around with it a bit and possibly even convert it so I get a wither skeleton spawner.

Also, I already know that if you spawn a skeleton in the nether it has an 80% chance to spawn as a wither skeleton.