Implement a set_mode method to Bulb to support “mono” mode

[Fredo70]

In the myStrom api documentation only two modes are documented: rgb and hsv. However, there is a third mode: mono. In this mode, the colour temperature can be set instead of the colour itself.
In your readme this is mentioned: https://github.com/home-assistant-ecosystem/python-mystrom/blob/master/README.rst#set-mono-white

Some precision: the color temperature for mono is a value between 1 and 18 where 1 = warmest, 18 = coldest.
The color should be: ColorTemparature;Brightness. For example 6;80