Added incrementBrightness support for groups - #417

Awesome! Thank you for doing this. Now how do we get @Foddy to merge it so we can get this in the easy Node Red installer?

@fjulian79 Could you implement decrementBrightness too?

@peterbaker maybe the name is misleading but you can use negative values too:
incrementBrightness +10 --> 10% brighter
incrementBrightness -10 --> 10% darker

So what you request is already there, it would be just another name and inverse logic:
decrementBrightness -10 --> 10% brighter
decrementBrightness +10 --> 10% darker

So I think this would not add a lot but would bloat the code.

Finally .. there is a fork of this repo which is way more agile, see https://github.com/mauricedominic/node-red-contrib-huemagic.git I have filed my pull request over there as well and it has been accepted. So maybe switching to mauricedominic is a valid option.

I did notice that increment works both ways after I wrote that, but also I still think it'd be useful (maybe just to me) to have decrementBrightness as that was how HueMagic 3 worked and including it would maintain existing flows.

Either way, just glad to see a path towards actually being able to use HueMagic 4. Thank you!

Also, it would then mirror the commands for individual lights (which have both incrementBrightness and decrementBrightness).

@peterbaker Ack, I agree! I have not had those background info's ;-) I will try to get it done in the next days.

@peterbaker I'm done .. I have tested decrementBrightness with my lamps, works fine from my point of view.