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.