Adds --cores option to limit to num_cores

[kdmukai]

Additional usage option:

  -c, --cores <NUM_CORES>
          Number of processor cores to use [default: 8]

note: actual default number of cores will depend on each machine

Validates against actual number of processor cores onboard. Must range from > 0 to <= num_cpus::get().

Rationale: Would be nice to keep grinding in the background while running other processor-heavy tasks (e.g. Docker) by keeping some cores free.

[grunch]

Great contribution! makes sense to have this, thanks