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.
How strongly do you feel about this? I agreed until I started implementing the rest. Having a default color of {red: 255, green: 255, blue: 255} simplifies the logic in .on(), etc. Otherwise I have to always check to see if previous color is defined. And I'll just set it to 255 across the board if it isn't anyway.