FEATURE: Customizable rules and plugins for PrettyText.markdown.

This commit extends the options which can be passed to
`PrettyText.markdown` so that which Markdown-it rules and Discourse
Markdown plugins to be used when rendering a text can be customizable.
Currently, this extension is mainly used by plugins.

app/assets/javascripts/discourse/tests/unit/lib/pretty-text-test.js

@@ -1493,6 +1493,45 @@ var bar = 'bar';
     );
   });
 
+  test("customizing markdown-it rules", function (assert) {
+    assert.cookedOptions(
+      "**bold**",
+      { markdownItRules: [] },
+      "<p>**bold**</p>",
+      "does not apply bold markdown when rule is not enabled"
+    );
+
+    assert.cookedOptions(
+      "**bold**",
+      { markdownItRules: ["emphasis"] },
+      "<p><strong>bold</strong></p>",
+      "applies bold markdown when rule is enabled"
+    );
+  });
+
+  test("features override", function (assert) {
+    assert.cookedOptions(
+      ":grin: @sam",
+      { featuresOverride: [] },
+      "<p>:grin: @sam</p>",
+      "does not cook emojis when Discourse markdown engines are disabled"
+    );
+
+    assert.cookedOptions(
+      ":grin: @sam",
+      { featuresOverride: ["emoji"] },
+      '<p><img src="/images/emoji/google_classic/grin.png?v=10" title=":grin:" class="emoji" alt=":grin:"> @sam</p>',
+      "cooks emojis when only the emoji markdown engine is enabled"
+    );
+
+    assert.cookedOptions(
+      ":grin: @sam",
+      { featuresOverride: ["mentions", "text-post-process"] },
+      `<p>:grin: <span class="mention">@sam</span></p>`,
+      "cooks mentions when only the mentions markdown engine is enabled"
+    );
+  });
+
   test("emoji", function (assert) {
     assert.cooked(
       ":smile:",

app/assets/javascripts/pretty-text/addon/engines/discourse-markdown-it.js

@@ -353,6 +353,12 @@ export function setup(opts, siteSettings, state) {
     }
   });
 
+  if (opts.featuresOverride) {
+    Object.keys(opts.features).forEach((feature) => {
+      opts.features[feature] = opts.featuresOverride.includes(feature);
+    });
+  }
+
   let copy = {};
   Object.keys(opts).forEach((entry) => {
     copy[entry] = opts[entry];
@@ -371,14 +377,22 @@ export function setup(opts, siteSettings, state) {
     enableDiffhtmlPreview: siteSettings.enable_diffhtml_preview,
   };
 
-  opts.engine = window.markdownit({
+  const markdownitOpts = {
     discourse: opts.discourse,
     html: true,
     breaks: !siteSettings.traditional_markdown_linebreaks,
     xhtmlOut: false,
     linkify: siteSettings.enable_markdown_linkify,
     typographer: siteSettings.enable_markdown_typographer,
-  });
+  };
+
+  if (opts.discourse.markdownItRules !== undefined) {
+    opts.engine = window
+      .markdownit("zero", markdownitOpts) // Preset for "zero", https://github.com/markdown-it/markdown-it/blob/master/lib/presets/zero.js
+      .enable(opts.discourse.markdownItRules);
+  } else {
+    opts.engine = window.markdownit(markdownitOpts);
+  }
 
   const quotation_marks = siteSettings.markdown_typographer_quotation_marks;
   if (quotation_marks) {

app/assets/javascripts/pretty-text/addon/pretty-text.js

@@ -38,6 +38,8 @@ export function buildOptions(state) {
     customEmojiTranslation,
     watchedWordsReplace,
     watchedWordsLink,
+    featuresOverride,
+    markdownItRules,
   } = state;
 
   let features = {};
@@ -76,6 +78,8 @@ export function buildOptions(state) {
     disableEmojis,
     watchedWordsReplace,
     watchedWordsLink,
+    featuresOverride,
+    markdownItRules,
   };
 
   // note, this will mutate options due to the way the API is designed