config: read tcpKeepAlive from file

[acoshift]

fixed #73

[codecov]

Codecov Report

Merging #97 into master will decrease coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   92.94%   92.64%   -0.31%     
==========================================
  Files          17       17              
  Lines         964      965       +1     
==========================================
- Hits          896      894       -2     
- Misses         51       54       +3     
  Partials       17       17

Impacted Files	Coverage Δ
app.go	86.17% <ø> (-2.44%)	⬇️
config.go	88.37% <100%> (+0.27%)	⬆️

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