Enable a site / local configuration file that is loaded automatically

In revising configuration files, it seems there are some 'global' options that all users should be using, but that depends on the local environment. The num_jobs is the prime example, as whether you are running on a personal machine or a HPC will greatly influence how this should be set.

I propose this be a file named site.cfg or local.cfg located in funpack/funpack/configs.

Unfortunately this would require a new option to, when necessary, suppress the sourcing of this configuration... something like --skip_site_config / -ssc.