refactor: remove marked.setOptions.highlight

[SukkaW]

This PR makes the code blocks result consistent by removing marked.setOptions.highlight.

More info can be found here: hexojs/hexo#4010 (comment)

config.highlight.enable: true

By default hljs is disabled and wrap is enabled. Hexo will render something likes:

<figure class="highlight sh">
  <table>
    ...
  </table>
</figure>

config.highlight.enable: false with hexo-renderer-marked

Since highlight has been disabled, the before_post_render filter will do nothing and leave everything to helpers (powered by nunjucks) and marked (from hexo-renderer-marked).

code() helper is designed to leave everything it should be without applying any highlight.js specific options, so it will render something like:

<pre>
  <code>
    ...
  </code>
</pre>

But for code block wrapped in backticks, it will be rendered by hexo-rendered-marked. And hexo-renderer-marked will call hexo-util.highlight with only str and lang option is given. So it will be rendered to:

<pre>
  <code class="bash">
    ...
  </code>
</pre>

config.highlight.enable: false with hexo-rendered-markdown-it

For code() helper, the result is pretty much the same as with hexo-renderer-marked (just as I said, helpers will not be processed by markdown renderer).

For code block wrapped in backticks, since markdown-it itself has no marked.setOptions.highlight like marked, so the result is the same:

<pre>
  <code>
    ...
  </code>
</pre>

[coveralls]

Coverage remained the same at 100.0% when pulling 2029738 on SukkaW:master into 7c7f513 on hexojs:master.

[curbengh]

To clarify, this PR is necessary to support hexojs/hexo-util#168? so that this plugin is neutral to the choice of syntax highlighting libraries?

[SukkaW]

@curbengh

This PR make sure that outputs are the same when config.highlight.enable is false.

hexo-renderer-markdown-it has no built-in syntax either.