Add config options for a command prefix/suffix

[camall3n]

This would be an easier way to handle things like:

• unbuffer [command]
• xvfb-run -a [command]
• singularity exec somefile.sif [command]

The suffix is useful if you want to pass the entire command into another command:

• singularity exec somefile.sif sh -c " [command] "
• ssh -f " [command] " > log.o 2> log.e < /dev/null &"