Add support for channels 2

[yakky]

• Add tests
• Add support for Django 3.0
• Update documentation

Fix #14

[codecov]

Codecov Report

Merging #15 into master will increase coverage by 3.03%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   94.18%   97.22%   +3.03%     
==========================================
  Files           3        3              
  Lines          86      108      +22     
  Branches        9        9              
==========================================
+ Hits           81      105      +24     
+ Misses          4        2       -2     
  Partials        1        1              

Impacted Files	Coverage Δ
knocker/signals.py	92.10% <83.33%> (-4.57%)	⬇️
knocker/consumers.py	100.00% <100.00%> (+26.66%)	⬆️
knocker/mixins.py	100.00% <100.00%> (ø)

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