!!spoiler!!plugin for markdown-it markdown parser
!!spoiler!! => <span class="spoiler">spoiler</span>
$ npm i @traptitech/markdown-it-spoiler
!!!spoiler!! => !<span class="spoiler">spoiler</span>
!!spoiler!!! => <span class="spoiler">spoiler!</span>
!!!spolier!! => <span class="spoiler">!spoiler</span>
!!spolier!!! => <span class="spoiler">spoiler</span>!