Add create_script option for job_loops

[lahuuki]

I find job_loop() to be very helpful in running array jobs with different options and appreciate the getopt() template provided.

However I often develop the base script first before implementing the loop. With create_shell=TRUE a new R script is created, that can overwrite the current work. It would be helpful for a similar create_script option. So when create_script=FALSE a script is not created and the getopt code is instead printed to output. The user could then copy that code in to the existing script.