Add colorloop effect for color lights

[slugzero]

When I tried to set up some effects for my color lights, I realized that there is no colorloop in the list of supported effects for most of them. Apparently, I am not the first one who is missing this (cf. Koenkk/zigbee2mqtt#1791). So here's my proposal for the implementation.

The change allows to trigger a continuous color loop with the command zigbee2mqtt/FRIENDLY_NAME/set with payload {"effect": colorloop}. This is in addition to the already-existing command hue_move and basically does the same.

As proposed in the feature request, it also accepts a transition value from 1...255 to set the time for a full loop, where 1 is shortest and 255 ist longest. I tested it with my lights and it roughly matches the seconds for a full loop, but this does not need to be the case for every light.
If no value for transition is given, a default value of 15 is used.

Example: for the slowest-possible color loop, publish to zigbee2mqtt/FRIENDLY_NAME/set with payload {"transition": 255, "effect":"colorloop"}. To turn off the color loop, publish {"effect":"stop_effect"}.

[Koenkk]

Thanks!