Support tri-backquote style code block and fix ordered list indent

[wetor]

Implement #386
testcase: test/backquote_code_style

Fix ordered list indent

<ol start="1">
  <li>
    <span>now with numbers</span>
  </li>
  <li>
    <span>the prisoner</span>
  </li>
  <ol start="1">
    <li>
      <span>not an <i>italic number</i></span>
    </li>
    <li>
      <span>a <b>bold human</b> being</span>
    </li>
  </ol>
  <li>
    <span>end</span>
  </li>
</ol>

now it will be correctly converted into

1. now with numbers
2. the prisoner
   1. not an italic number
   2. a bold human being
3. end

testcase : test/mixed_nested_lists

And, regarding the CLI parameter names and variable names in tri-backquote style, I didn't have a good name, so I used backquote_code_style. What do you think?

[wetor]

The implementation still has room for optimization, and discussion is welcome.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.42%. Comparing base (2723f84) to head (5ea5c32).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
+ Coverage   97.36%   97.42%   +0.06%     
==========================================
  Files          11       11              
  Lines        1137     1164      +27     
==========================================
+ Hits         1107     1134      +27     
  Misses         30       30              

Flag	Coverage Δ
unittests-3.10	97.42% <100.00%> (+0.06%)	⬆️
unittests-3.11	97.42% <100.00%> (+0.06%)	⬆️
unittests-3.12	97.42% <100.00%> (+0.06%)	⬆️
unittests-3.13	97.42% <100.00%> (+0.06%)	⬆️
unittests-3.9	97.42% <100.00%> (+0.06%)	⬆️

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[zeMinng]

Hello author, this function is very practical, I used today, but there is no formal package, I hope to merge and release soon