Add use method to extend markdown-it parser by plugin

[yhatt]

Added Marpit.use() instance method to extend markdown-it parser by plugin.

It is exactly same meaning as Marpit.markdown.use(), but it returns Marpit instance as chainable. It becomes easy to customize Marpit (Marp Core) especially in a functional engine implemented in Marp CLI (marp-team/marp-cli#42).

Usage in Marp Core / Marp CLI

// marp.config.js
const { Marp } = require('@marp-team/marp-core')
const container = require('markdown-it-container')
const mark = require('markdown-it-mark')

module.exports = {
  engine: opts => new Marp(opts).use(container, 'columns').use(mark),
}

<style>.columns { column-count: 2; }</style>

# Extend parser by ==markdown-it plugin==

::: columns
Lorem ipsum dolor sit amet, nec ex illud viderer vivendo, mea semper deleniti cu. His in brute justo. Vim facilisis dissentiet ei. Per cu efficiendi theophrastus, te pro dicta mucius, ut sed adhuc ponderum. No iriure convenire pri, ius at possit appareat disputando, mei ei libris oportere.
:::

markdown-it-mark is using in the pre-released Marp but we are not planned to adopt in Marpit and Marp Core by not-standard syntax. markdown-it-container is also not adopted by the same reason.

However, this customization will help needed user. The example of multi column is one of the alternative way to r​esolve yhatt/marp#82.

ToDo

• Add test