Make the exposed service configurable

[AlexGustafsson]

Backstory

There's a need to allow users to specify what service is exposed by this plugin to allow for other icons in the Home app etc. (see #122). For historical reasons, it has always been a simple switch - but it could've been anything that represents an on or off state and allows for switching between the states (such as a lamp, a speaker or a TV).

Issue

Implement a way to make the exposed service (currently a switch) configurable to allow for more accessory types supporting an on / off state, such as a lamp, speaker or TV.

The various accessories are documented here https://developers.homebridge.io.

Examples:

• https://developers.homebridge.io/#/service/AirPurifier
• https://developers.homebridge.io/#/service/Fan
• https://developers.homebridge.io/#/service/Lightbulb
• https://developers.homebridge.io/#/service/Outlet
• https://developers.homebridge.io/#/service/Speaker
• https://developers.homebridge.io/#/service/Television

[AlexGustafsson]

It would also be nice to be able to turn the accessory into a stateless button. See #183.