feat: Provide code fragments feature (#748)

Author: timaschew

docs/_media/example.js

@@ -0,0 +1,16 @@
+import fetch from 'fetch'
+
+const URL = 'https://example.com'
+const PORT = 8080
+
+/// [demo]
+const result = fetch(`${URL}:${PORT}`)
+  .then(function(response) {
+    return response.json();
+  })
+  .then(function(myJson) {
+    console.log(JSON.stringify(myJson));
+  });
+/// [demo]
+
+result.then(console.log).catch(console.error)

docs/embed-files.md

@@ -39,6 +39,21 @@ You will get it
 
 [filename](_media/example.md ':include :type=code')
 
+## Embedded code fragments
+Sometimes you don't want to embed a whole file. Maybe because you need just a few lines but you want to compile and test the file in CI.
+
+```markdown
+[filename](_media/example.js ':include :type=code :fragment=demo')
+```
+
+In your code file you need to surround the fragment between `/// [demo]` lines (before and after the fragment).  
+Alternatively you can use `### [demo]`.
+
+Example:
+
+[filename](_media/example.js ':include :type=code :fragment=demo')
+
+
 ## Tag attribute
 
 If you embed the file as `iframe`, `audio` and `video`, then you may need to set the attributes of these tags.

src/core/render/compiler.js

@@ -164,6 +164,7 @@ export class Compiler {
         embed = compileMedia[type].call(this, href, title)
         embed.type = type
       }
+      embed.fragment = config.fragment
 
       return embed
     }

src/core/render/embed.js

@@ -20,6 +20,11 @@ function walkFetchEmbed({embedTokens, compile, fetch}, cb) {
           if (token.embed.type === 'markdown') {
             embedToken = compile.lexer(text)
           } else if (token.embed.type === 'code') {
+            if (token.embed.fragment) {
+              const fragment = token.embed.fragment
+              const pattern = new RegExp(`(?:###|\\/\\/\\/)\\s*\\[${fragment}\\]([\\s\\S]*)(?:###|\\/\\/\\/)\\s*\\[${fragment}\\]`)
+              text = ((text.match(pattern) || [])[1] || '').trim()
+            }
             embedToken = compile.lexer(
               '```' +
                 token.embed.lang +