Add callback support

[j5shi]

Is there any method to add a callback function to vimproc#system_bg so that we can know the task running in background has been finished, or aborted as something wrong happened.

Or simply print something in Vim when background task is finished.

[Shougo]

Not implemented the feature.

[j5shi]

Thanks.

[Shougo]

But the feature request is very low priority.

[j5shi]

No problem.