Allow user code to set W element of RGBW pixels via pack() function (issue #25)

This is NOT True and Robust RGBW Handling™, but does allow easy setting of the W element of RGBW NeoPixels without having to unpack() a previously-pack()ed result. The pack() function accepts an optional W value, in which case it returns a 4-tuple that can be used with the NeoPixel() setter. This is esoteric but the need for it is discussed in issue #25, due to the way the CircuitPython NeoPixel setter works (tuples are preferred to packed ints). This provides a relatively benign quick compromise without having to implement a whole CRGBW class.