Autodetection of free port

[JoergVanAken]

First of all: voila is just great!

Last week I deployed a little application which the user can easily start by double clicking a windows batch file. This works fine but there is one little thing: The app cannot be started in multiple windows because the port number is fixed. I fixed this on my own by looping over a a few port numbers in the calling batch script. But when I start a new jserver a free port is autodetected. I would love to see this feature in voila, too.

[jtpio]

Thanks @JoergVanAken, that would indeed be very useful.

This is something that should normally come for free once #270 lands.

In the meantime, we could replicate the logic of the notebook server in voila.