Add support for working directory cleanup

[adamkewley]

Currently, Jobson will leave the working directories intact after a job has finished. This is useful for debugging the system. However, some of the jobs we run internally can produce >20 GB of data, which might still sit around after Jobson copies it to the final output location.

I usually use a separate cronjob to delete older working directories. However, if there's a lot of requests in a short period of time the disk will just fill up before the cronjobs get a chance to clean everything up.

For this fix I will:

• Add a cleanup_working_directories: [false] property into the config. Default behavior will be to not cleanup the directories, in order to maintain backwards compat.
• Make the job pipeline use that config property to cleanup working dirs

[adamkewley]

This has now been implemented in the master branch