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.

The indicator was not halfway when using 0.5 as the output value.

Also added an example of how the output could be wrapped in a
light component, to expose it to Home Assistant as a monochromatic
light. Not that useful probably, but a nice display of ESPHome's
flexibility.

The example.yaml now also updates the brightness indicator when
the light brightness changes. When switching the device to night
light mode, it will turn off the front panel illumation completely.

And last but not least: the example configuration now shows a way
to set the device to a random color when touching the color button.

Update example.yaml

Add 'id: ${name}' to light section of the example configuration

Create LICENSE.md