Pandoc-filter

On Working...

- Modify `md2html_hash_anchor_and_internal_link_filter` to add an invisible link for each header.
- Now, for each header, an invisible link that links to the header itself will be added to it.
- The added invisible link will be assigned the class of `headerlink`.
- This will be useful for generating anchors by some renderers/frameworks/tools, such as [hexo-theme-butterfly](https://butterfly.js.org/posts/4aa8abbe/?highlight=%25+endnote#%E9%A0%81%E9%9D%A2%E9%8C%A8%E9%BB%9E).

- Modify `md2md-norm-footnote-filter`. Now, it deals with the footnote content as follows:
- Remove unnecessary `\n`.
- Remove the markdown link.
- Normalize many markdown formats like emphasis(italic) and strong(bold).

- Add `md2html-enhance-footnote-filter`. This filter will enhance the footnote content as follows:
- upgrade the link-like string to a `link` element.
- Remove unnecessary `\n`.
- Remove the markdown link.
- Normalize many markdown formats like emphasis(italic) and strong(bold).

- Modify `md2html_hash_anchor_and_internal_link_filter` to handle internal links within hexo tag plugins.
- Now, it will compulsorily modify internal_link's url, even though its target is not found.

Add `md2html_increase_header_level_filter`, which will increase the header level by 1.

Remove more unnecessary type-checking clauses for accelerating.

Remove unnecessary type-checking for accelerating.