Add a way to configure SLURM `#SBATCH --output`

### Proposed change

It would be nice to allow the configuration of the `--output` sbatch parameter the same way as for other parameters.
This would enable customisation of the output file as well as passing `/dev/null` to silent SLURM output.

### Alternative options

Leave it as it is: `--output={{homedir}}/jupyterhub_slurmspawner_%j.log`

### Who would use this feature?

I'm already using a similar feature in [jupyterhub_moss](https://github.com/silx-kit/jupyterhub_moss/), a customized SLURM Spawner (See https://github.com/silx-kit/jupyterhub_moss/blob/fbcf07c7b7198af97e8cd3545458759e5e882b9e/jupyterhub_moss/batch_script.sh#L13), but this requires duplicating the `batch_script`.

### (Optional): Suggest a solution

Add a `req_output` to `SlurmSpawner` (see #258).