Allow to pass functions to the `end_command` in extensions

[jvolker]

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.

[jmwohl]

Yeah, I don't see a reason not to support this... feel free to submit a PR!