Support init message

[derom]

Hey! First of all, thanks for this repo.
It would be nice to have the ability to send a message to websocket server. For example, subscribe to a specific channel of messages before starting a test.

[coveralls]

Coverage decreased (-0.01%) to 75.915% when pulling 83dd683 on derom:support-init-message into f7c91eb on Fitblip:master.

[codecov-io]

Codecov Report

Merging #19 into master will decrease coverage by 0.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   75.92%   75.91%   -0.02%     
==========================================
  Files           5        5              
  Lines         324      328       +4     
==========================================
+ Hits          246      249       +3     
- Misses         78       79       +1

Impacted Files	Coverage Δ
wsstat/main.py	91.66% <100%> (+0.36%)	⬆️
wsstat/clients.py	70.58% <66.66%> (-0.07%)	⬇️

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[derom]

There is no "WebSocket connection is closed" anymore python-websockets/websockets@8d907e0#diff-edaffe2a8ed181f48999c520ad93d60cL17
so I changed the expected message

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

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Files	Patch %	Lines
wsstat/clients.py	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##             master      #19   +/-   ##
=========================================
  Coverage          ?   75.91%           
=========================================
  Files             ?        5           
  Lines             ?      328           
  Branches          ?        0           
=========================================
  Hits              ?      249           
  Misses            ?       79           
  Partials          ?        0           

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