Added --html option to generate HTML report

[vsapronov]

This is very useful when running locust in headless mode.

[codecov]

Codecov Report

Merging #1611 into master will decrease coverage by 0.01%.
The diff coverage is 90.38%.

@@            Coverage Diff             @@
##           master    #1611      +/-   ##
==========================================
- Coverage   82.69%   82.67%   -0.02%     
==========================================
  Files          28       29       +1     
  Lines        2565     2586      +21     
  Branches      390      391       +1     
==========================================
+ Hits         2121     2138      +17     
- Misses        349      354       +5     
+ Partials       95       94       -1     

Impacted Files	Coverage Δ
locust/main.py	21.17% <20.00%> (-0.03%)	⬇️
locust/html.py	97.72% <97.72%> (ø)
locust/argument_parser.py	77.39% <100.00%> (+0.19%)	⬆️
locust/web.py	90.38% <100.00%> (-1.18%)	⬇️
locust/user/task.py	96.27% <0.00%> (-0.54%)	⬇️
locust/runners.py	85.96% <0.00%> (+0.38%)	⬆️

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[cyberw]

Cool stuff! Can you add a test case?

[vsapronov]

Cool stuff! Can you add a test case?

Hi, I think the functionality that I have extracted is under tests that are hitting /stats/report here: https://github.com/locustio/locust/blob/master/locust/test/test_web.py#L300-L345.

[cyberw]

Can you add a "real" test in test_main.py ? I agree that some of the underlying features are tested, but not the parsing of the parameter, writing to file etc.

[cyberw]

@vsapronov ?

[cyberw]

replaced by #1637