feat(index): add <import src="./file.html"> (HTML Imports) support (options.import)

Author: michael-ciniawsky

src/lib/plugins/import.js

@@ -0,0 +1,44 @@
+/* eslint-disable */
+// External URL (Protocol URL)
+const TEST_URL = /^\w+:\/\//;
+
+// TODO(michael-ciniawsky)
+// add filter method for urls (e.g `options.import`) (#158)
+const filter = (url, options) => {
+  return TEST_URL.test(url) || url.startsWith('//');
+}
+
+export default function (options = {}) {
+  return function (tree) {
+    let idx = 0;
+    const imports = {};
+
+    tree.match([ { tag: 'import' }, { tag: 'include' } ], (node) => {
+      if (node.attrs && node.attrs.src) {
+        // Remove <import>/<include> tag
+        node.tag = false;
+
+        // TODO(michael-ciniawky)
+        // add warning about invalid use of external urls within <import> (#?)
+
+        // Ignore external && filtered urls
+        if (filter(node.attrs.src, options)) return;
+        // Add url to messages.imports
+        imports[`HTML__IMPORT__${idx}`] = node.attrs.src;
+        // Add content placeholders to HTML
+        node.content = options.template
+          ? '${' + `HTML__IMPORT__${idx}(${options.template})` + '}'
+          : '${' + `HTML__IMPORT__${idx}` + '}';
+
+        idx++;
+      }
+
+      return node;
+    });
+
+    // Add imports to result.messages
+    tree.messages.push(imports)
+
+    return tree;
+  };
+}