Implement device factory

[rytilahti]

This will make it simple for downstream users to construct device instances for all supported devices given only the host and its token.

All device subclasses register themselves automatically to the factory. The create(host, token, model=None) class method is the main entry point to use this.

Supersedes #1328
Fixes #1117

[codecov-commenter]

Codecov Report

Merging #1556 (db807c0) into master (dc55eba) will decrease coverage by 0.00%.
The diff coverage is 83.60%.

@@            Coverage Diff             @@
##           master    #1556      +/-   ##
==========================================
- Coverage   81.05%   81.05%   -0.01%     
==========================================
  Files         153      154       +1     
  Lines       15024    15073      +49     
  Branches     3680     3697      +17     
==========================================
+ Hits        12178    12217      +39     
- Misses       2599     2609      +10     
  Partials      247      247              

Impacted Files	Coverage Δ
miio/devicefactory.py	80.76% <80.76%> (ø)
miio/click_common.py	52.68% <100.00%> (+0.46%)	⬆️
miio/integrations/light/yeelight/spec_helper.py	100.00% <100.00%> (ø)
.../light/yeelight/tests/test_yeelight_spec_helper.py	100.00% <100.00%> (ø)
miio/integrations/light/yeelight/yeelight.py	88.39% <100.00%> (+1.16%)	⬆️
...integrations/light/yeelight/tests/test_yeelight.py	95.31% <0.00%> (-1.28%)	⬇️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more