Extract alt text from images; add "skip HTML" feature

[jameslittle230]

Closes #148 and closes #186 by replacing the HTML parsing library to one that gives me more control over what part of an HTML page I'm looking at.

Starts foundational work for #159 (in that we can retrieve nearest-ID data now), but to close that issue still requires some JS changes to actually link to those IDs.

[codecov]

Codecov Report

Merging #183 (45e11a0) into master (05a0424) will increase coverage by 1.62%.
The diff coverage is 96.21%.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
+ Coverage   61.38%   63.00%   +1.62%     
==========================================
  Files          54       54              
  Lines        1893     1987      +94     
  Branches       95       95              
==========================================
+ Hits         1162     1252      +90     
- Misses        731      735       +4     

Flag	Coverage Δ
javascript	45.58% <ø> (ø)

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Impacted Files	Coverage Δ
src/config/file.rs	69.23% <ø> (ø)
src/config/mod.rs	81.81% <ø> (ø)
...s/word_list_generators/html_word_list_generator.rs	95.55% <95.57%> (-2.41%)	⬇️
src/index_versions/v3/builder/errors.rs	38.23% <100.00%> (+8.23%)	⬆️
...3/builder/fill_intermediate_entries/frontmatter.rs	93.33% <100.00%> (-0.22%)	⬇️
...rsions/v3/builder/fill_intermediate_entries/mod.rs	88.99% <100.00%> (+0.10%)	⬆️
...rd_list_generators/markdown_word_list_generator.rs	100.00% <100.00%> (ø)
src/index_versions/v3/builder/mod.rs	97.26% <100.00%> (+1.54%)	⬆️

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