Improve background job.nu

[WindSoilder]

It contains the following changes:

1. add Q&A session in README
2. add --label, --group flag to spawn custom command, so we can run background job in a more flexible way.
3. add output custom command, so we can view job's output message easily
4. add --detailed flag to status command, so we can have a chance to view status in a simpler view

[fdncred]

Thanks. I also think that looking at this script can give us an idea of how we'd write our own someday.