Register the plugin as platform

[afharo]

Resolves #247

Registering the plugin as platform allows dynamic discovery of the services. Apart from stopping the connection errors we typically see when starting up, it opens the door to future improvements like:

1. Query the robot before creating the services: a real room discovery feature?
2. Allow other forms of authentication (Xiaomi Cloud could retrieve the token automatically based on user and password).
3. Identify the model and the speeds before creating the services.

TODO:

• Update docs
• Update config schema
• Add/Update tests

[codecov]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (ec81661) 50.65% compared to head (abe1cf8) 51.04%.

Files	Patch %	Lines
src/homebridge_entities/platform_accessory.ts	0.00%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #855      +/-   ##
==========================================
+ Coverage   50.65%   51.04%   +0.38%     
==========================================
  Files          58       62       +4     
  Lines        1516     1573      +57     
  Branches      221      231      +10     
==========================================
+ Hits          768      803      +35     
- Misses        674      696      +22     
  Partials       74       74              

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