Adds isOn property to RGB

Adds isOn property for RGB that looks to see if ANY of the colors are
on. When calling .on(), it leaves the led as-is if it’s already on.

Also adds chaining capability to .on() and .off() and saves color state
when those functions are called.

Does not yet address saving previous color as requested in #375

Other functions like toggle/brightness/etc still need to be updated to
treat RGB as a whole rather than individual LEDs.
I took a shortcut since I'm not saving color state yet. But you're right, I should go ahead and store a reference to the leds in state and check their isOn property instead.