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 83dd683ba42a1bc62606fda875514a716443f544 on derom:support-init-message into f7c91eb8765a801ad5e0504ca22ee2ec832cf682 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%)	:arrow_up:
wsstat/clients.py	70.58% <66.66%> (-0.07%)	:arrow_down:

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[derom]

There is no "WebSocket connection is closed" anymore https://github.com/aaugustin/websockets/commit/8d907e029996a5563ceb5b65e02406c442674733#diff-edaffe2a8ed181f48999c520ad93d60cL17 so I changed the expected message