Fixes #10
"When changing RGB color settings, while night light mode is active,
transitions don't look good. On many of the transitions, the light
level shows a significant drop."

This commit fully suppresses transitions when the light is in
night light mode.

Documentation and binary sensor code quality improvements.

Implemented support for presets.

Some people wanted to buy me beer, coffee and the like as a thank you for the work that I put into the ESPHome firmware code for the Xiaomi Mijia Bedside Lamp 2. Well, I don't say no to a beer, cheers! ;-)

The reasoning behind this can be found in this issue:
https://github.com/mmakaay/esphome-yeelight_bs2/issues/7

When you are already using a previous version of the firmware,
then note that the following changes are necessary:

* Update to the latest version of the custom component code.
* Rename your folder `custom_components/yeelight_bs2` to
  `custom_components/xiaomi_bslamp2`
* Update your yaml config by changing all occurrences of `yeelight_bs2`
  into `xiaomi_bslamp2`

The links to the github repository will remain functional after
the renaming operation, so it's not required to grab a fresh clone.

For example: `id(${id_light_mode}).state == "night"`
This is used in the example.yaml to turn off the illumination
of the front panel, when the lamp goes into night light mode.