v0.4 - Config revamp

Thanks to some suggestions and help from @OttoWinter in the ESPHome Discord, the configuration has been simplified / tidied up.

Improvements to the config:

• All sensors are now optional
• It's now easier to choose which sensors to use
  • Volume and Tune Playing sensors now have their own config blocks (not mixed with Button X Tune sensors any more)
  • Button X binary_sensors and Button X Tune sensors now have simplified lambdas, which cover any number of sensors