Add keepalive flag. Fixes issue #30

[cvhariharan]

Adds a new boolean flag -K or --keepalive to use persistent connections for the requests.

[codecov-commenter]

Codecov Report

Merging #43 into master will decrease coverage by 0.33%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
- Coverage   72.07%   71.75%   -0.32%     
==========================================
  Files           8        8              
  Lines         358      361       +3     
==========================================
+ Hits          258      259       +1     
- Misses         82       84       +2     
  Partials       18       18              

Impacted Files	Coverage Δ
attacker/attacker.go	69.45% <0.00%> (-1.98%)	⬇️
main.go	55.92% <88.89%> (-0.12%)	⬇️

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[nakabonne]

@cvhariharan Hey, thank you for your contribution!