Allow passing markdown-it instance as markdown constructor option

[yhatt]

We are organizing confused features. This PR will allow passing markdown-it instance as markdown constructor option, to wrap any instance by Marpit.

const markdownIt = new MarkdownIt()
const marp = new Marp({ markdown: markdownIt })

As before, you can also pass markdown-it constructor options, because it considers need for YAML configuration by Marp CLI.

markdownItPlugins interface had difficult usage is now deprecated. It as same meaning as using added constructor option.