htmlindex: support img srcset attribute

[cornelk]

Close #33

[codecov]

Codecov Report

Attention: Patch coverage is 87.09677% with 4 lines in your changes missing coverage. Please review.

Project coverage is 43.34%. Comparing base (2ddb00f) to head (4317e64).

Files	Patch %	Lines
htmlindex/htmlindex.go	87.09%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
+ Coverage   42.16%   43.34%   +1.17%     
==========================================
  Files          14       14              
  Lines         721      736      +15     
==========================================
+ Hits          304      319      +15     
- Misses        365      366       +1     
+ Partials       52       51       -1     

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