add coverage style html report

[cleder]

Add a (quick and dirty) coverage style HTML report.
#193
The generated HTML still needs some styling, but is usable.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 4.16667% with 23 lines in your changes missing coverage. Please review.

Project coverage is 80.32%. Comparing base (afe4db7) to head (5f89005).

Files with missing lines	Patch %	Lines
mutmut/cache.py	4.16%	23 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
- Coverage   81.44%   80.32%   -1.12%     
==========================================
  Files           7        7              
  Lines        1633     1657      +24     
==========================================
+ Hits         1330     1331       +1     
- Misses        303      326      +23     

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[boxed]

How does this improve on the existing report?

[cleder]

it shows every line with its line number in the source code.
If the line has surviving mutants or suspicious mutants, they will be shown below the original line with their emoji

[cleder]

FYI: that is what I use for cleder/pygeoif#19