Expects a string and returns the modified label to display.
Additionally, the attribute `data-heading-label` may be used on a heading to specify a shorter string to be used in the TOC.
data-heading-label
을 통해 변경하면 된다!!!!
예시
<h3 id="it-is-never-displayed" data-heading-label="only for toc">only for post</h3>