fix(embed): provide fallback with esi:remove when esi include fails

Until now, a failed ESI include would fail silently, without a possible fallback. This change falls
back to the markdown rendered as HTML in case the ESI does not work, giving at least an
approximation of the content created.

fixes #267

src/utils/embed-handler.js

@@ -16,13 +16,20 @@
 const URI = require('uri-js');
 
 function embed({ EMBED_SERVICE }) {
-  return function handler(h, node) {
+  return function handler(h, node, _, handlechild) {
     const { url } = node;
     const props = {
       // prepend the embed service for absolute URLs
       src: (URI.parse(url).reference === 'absolute' ? EMBED_SERVICE : '') + url,
     };
-    const retval = h(node, 'esi:include', props);
+    const retval = [h(node, 'esi:include', props)];
+
+    if (node.children && node.children.length) {
+      const rem = h(node, 'esi:remove', {});
+      node.children.forEach(childnode => handlechild(h, childnode, node, rem));
+      retval.push(rem);
+    }
+
     return retval;
   };
 }

test/testEmbedHandler.js

@@ -122,6 +122,7 @@ https://www.youtube.com/watch?v=KOxbO0EI4MA
     assert.equal(result.response.body, `<p>Hello World
 Here comes an embed.</p>
 <esi:include src="https://example-embed-service.com/https://www.youtube.com/watch?v=KOxbO0EI4MA"></esi:include>
+<esi:remove><p><a href="https://www.youtube.com/watch?v=KOxbO0EI4MA">https://www.youtube.com/watch?v=KOxbO0EI4MA</a></p></esi:remove>
 <p><img src="easy.png" alt="Easy!" srcset="easy.png?width=480&amp;auto=webp 480w,easy.png?width=1384&amp;auto=webp 1384w,easy.png?width=2288&amp;auto=webp 2288w,easy.png?width=3192&amp;auto=webp 3192w,easy.png?width=4096&amp;auto=webp 4096w" sizes="100vw"></p>`);
   });
 
@@ -152,6 +153,7 @@ Here comes an embed.
     assert.equal(result.response.body, `<p>Hello World
 Here comes an embed.</p>
 <esi:include src="/test/foo.embed.html"></esi:include>
+<esi:remove><p>./foo.md</p></esi:remove>
 <p><img src="easy.png" alt="Easy!" srcset="easy.png?width=480&amp;auto=webp 480w,easy.png?width=1384&amp;auto=webp 1384w,easy.png?width=2288&amp;auto=webp 2288w,easy.png?width=3192&amp;auto=webp 3192w,easy.png?width=4096&amp;auto=webp 4096w" sizes="100vw"></p>`);
   });
 });