Document adding markdown-it plugins

[Manvel]

ex.:

// Open external links in the new tab
const markdownLink = require("markdown-it-link-attributes");
const markdownOptions = {
  plugins: [
    [
      markdownLink, {
        pattern: /^https?:\/\//,
        attrs: {
          target: "_blank",
          rel: "noopener"
        }
      }
    ]
  ]
};

module.exports = {markdownOptions};

Can be either array of parameters with plugin as the first parameter or a plugin import.

Relevant links:

CMintS issue -> https://github.com/cmints/cmints/issues/186 Markdown-it documentation -> https://github.com/markdown-it/markdown-it#plugins-load

[Manvel]

See -> https://github.com/cmints/cmints/issues/190