CI maintenance and adding testing to #118

[klauer]

• CI version bump
• blark.html.format_file_as_html helper (users should use blark format --output-format html)
• Explicit html tests

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.12281% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.6%. Comparing base (1922eed) to head (54632b9).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
blark/tests/test_html.py	92.8%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #120     +/-   ##
========================================
+ Coverage    82.4%   82.6%   +0.1%     
========================================
  Files          27      28      +1     
  Lines        5897    5958     +61     
========================================
+ Hits         4862    4922     +60     
- Misses       1035    1036      +1     

Files with missing lines	Coverage Δ
blark/format.py	74.4% <100.0%> (ø)
blark/html.py	100.0% <100.0%> (ø)
blark/tests/test_html.py	92.8% <92.8%> (ø)

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