Inject markdown headers as page attributes #51

Add a markdown-it plugin to inject markdown headers [{ text: 'Hello', slug: hello' }] as page.attributes.markdownHeaders, similar to Vuepress's this.$page.headers.

When to inject:

• page.attributes.injectMarkdownHeaders === true
• Or page.attributes.injectMarkdownHeaders !== false && api.config.markdown.injectMarkdownHeaders

The injected markdown headers can be used by https://github.com/egoist/saber/pull/49