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.