Added --html option to save HTML report

[rloomans]

This is a fork of @vsapronov's PR #1611, with the addition of a test.

[codecov]

Codecov Report

Merging #1637 (f9854dd) into master (03b6df4) will decrease coverage by 0.14%.
The diff coverage is 90.38%.

@@            Coverage Diff             @@
##           master    #1637      +/-   ##
==========================================
- Coverage   80.57%   80.42%   -0.15%     
==========================================
  Files          29       30       +1     
  Lines        2656     2677      +21     
  Branches      410      411       +1     
==========================================
+ Hits         2140     2153      +13     
- Misses        419      427       +8     
  Partials       97       97              

Impacted Files	Coverage Δ
locust/main.py	20.25% <20.00%> (-0.01%)	⬇️
locust/html.py	97.72% <97.72%> (ø)
locust/argument_parser.py	77.58% <100.00%> (+0.19%)	⬆️
locust/web.py	90.38% <100.00%> (-1.18%)	⬇️
locust/user/task.py	95.74% <0.00%> (-0.54%)	⬇️
locust/runners.py	84.80% <0.00%> (-0.39%)	⬇️

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[cyberw]

thanks! (and thanks @vsapronov)