coreIndex fails || when set to the first core 0

[jonsmith1982]

When opts.coreIndex is set to 0 it does not check the first cores usage as it fails on line 36 and defaults to -1 returning the average of all cores...

opts.coreIndex = opts.coreIndex || -1;

I would also like it to return the coreIndex value if present from the callback function so you know which one your dealing with at that particular moment.

I shall issue a separate patch for each if this is alright with yourself?

Regards,
Simon

[jonsmith1982]

opts.coreIndex = opts.coreIndex !== undefined ? opts.coreIndex : -1;

This is what I replaced it with. I'm no expert so there maybe a better way to do it.