Return a new numeral on math operations

[sobrinho]

Hi,

Would be great to return a new numeral instance on math operations like this:

var value = numeral(100),
    discount = numeral(0);

return value.subtract(value.multiply(discount).divide(numeral(100)));

It will return 0 instead of 90 since the value.multiply(discount) will change the value instance to 0 instead of returning a new instance with 0.

The idea is to mutate the numeral instance only using numeral.fn.set instead of mutating the called instance.

WDYT?

[adamwdraper]

All other numeral functions manipulate the instance so I don't think these should act differently.