Support functional engine

[yhatt]

This PR will support the functional engine by passing engine option through JS configuration. It would become to customize Marp or Marpit engine easily by a lot of markdown-it plugins.

For example, you can use ==mark== syntax by markdown-it-mark. It will turn into <mark>mark</mark>.

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

module.exports = {
  engine: opts => new Marp(opts).use(markdownItMark)
}