Support custom MarkBind plugins

[jamos-tay]

Edited from #467 (comment)

Consider using a plugin concept, e.g. in Eslint.

For a start, we can have a postRender entry point. This means something like this in site.json:

{
  "plugins": [
    "custom-plugin"
  ],
  "context": { // Can consider a different name
    "custom-plugin": ... data
  }
}

And in src/plugins/custom-plugin.js:

module.exports = {
  postRender: function (content, siteContext, pageConfig) {
    // ...
    return $.html();
  },
};

And in Page.js:

+ .then(result => this.postRender(result, siteContext, pageConfig))

Page.prototype.postRender = function (content, siteContext, pageConfig) {
  let postRenderedContent = content;
  plugins.forEach((plugin) => { // Might be able to do `reduce` instead of `forEach`
    if (plugin.postRender) {
      postRenderedContent = plugin.postRender(postRenderedContent, siteContext, pageConfig);
    }
  });
  return postRenderedContent;
};

Entry points we can support (for now):

      .then(result => addContentWrapper(result))

      -- prerender entry point--

-     .then(result => this.insertSiteNav((result)))
-     .then(result => this.insertFooter(result)) // Footer has to be inserted last to ensure proper formatting
-     .then(result => formatFooter(result))
      .then(result => markbinder.resolveBaseUrl(result, fileConfig))
      .then(result => fs.outputFileAsync(this.tempPath, result))
      .then(() => markbinder.renderFile(this.tempPath, fileConfig))

      -- postrender entry point--

-     .then(result => this.filterTags(this.frontMatter.tags, result))
-     .then(result => this.addAnchors(result))

Consider migrating to plugins?

• Site nav and footer
• Tags
• Anchors

[yamgent]

Consider migrating to plugins?

• Site nav and footer
• Tags
• Anchors

They can be considered, but since this will just involve mostly code movement with no visible changes/no new features for the user, the priority for this issue will be low.