Expose undertow thread configurations

[pds2208]

It would be very useful to expose undertow's setWorkerThreads() and setIoThreads() methods in cask.Main()

[lihaoyi]

Currently you can do that by simply overriding cask.main.Main.main, which handles all the undertow initialization. Not super elegant, but in theory it should expose everything that's possible to do with the undertow server