Made the lamp work with ESPHome 2021.10.0

Thanks Jos!

* Updated documentation for 1.18.0 release with external components support.

* Moved example.yaml to the root dir, in line with the recommendations from the ESPHome external components guide.

* Normalized the version numbering for ESPHome references.

* Fixed the styling of the example.yaml references.

Co-authored-by: Maurice Makaay <mmakaay1@xs4all.net>

Implemented support for presets.

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.