Add escaped commands

[dawbarton]

Add the (ASCII only) list of escaped special characters from https://en.wikibooks.org/wiki/LaTeX/Special_Characters as symbols. Namely, "_ % $ # &".

Fixes #127

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.03%. Comparing base (819bbec) to head (6b31d00).
⚠️ Report is 37 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
- Coverage   78.39%   74.03%   -4.36%     
==========================================
  Files          10       10              
  Lines         597      647      +50     
==========================================
+ Hits          468      479      +11     
- Misses        129      168      +39     

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[Kolaru]

Thanks a lot!