Add pre-run and post-run scripting option

[barjac]

Add an option to enable and specify scripts to run from within klog before and after the main program starts/ends in order allow custom housekeeping.

The scripts and the config for this to be stored outside ~/.klog/ maybe a ~/.klogscripts/ folder. This would then allow for the case where ~/.klog is a mount point, not a local directory, which may or may not have a remote location mounted when the scripts are run.

In the case of a pre-run script, klog must abort if this script returns >= 1.