Allow to pass functions to the end_command in extensions #43

Sometimes it's necessary to do some cleanup tasks after finishing to show some artwork.

While it's currently possible to run a shell script by passing a string via the end_command property, it would be nice to do this directly in Node.Js by passing a function instead. Just like it's possible forstart_command.

I had a quick look into the code, and it seems it's only necessary to copy a few lines of code to allow this.

Thanks.