feat: Add transform function to selector

src/extractors/custom/README.md

@@ -71,6 +71,34 @@ export const ExampleExtractor = {
     ...
 ```
 
+If the array includes a callback function as 3rd element, all the results pass through that transformer. Returned values are the result of selector.
+For example:
+
+```javascript
+date_published: {
+    selectors: [[
+        'meta[name="article:published_time"]',
+         'value',
+          (item) => moment.from(convertNumbersToEnglish(item), 'fa',
+            'YYYY/MM/DD - HH:mm').toISOString();
+    ]]
+}
+```
+
+```javascript
+
+
+extend: {
+  tags: {
+    selectors: [[
+        'meta[name=\'article:tag\']', 'value', (item) => {
+        return item.split(/\s*,\s*/);
+      }]],
+    allowMultiple: true
+  }
+}
+```
+
 This is all you'll need to know to handle most of the fields Mercury parses (titles, authors, date published, etc.). Article content is the exception.
 
 #### Content selectors

src/extractors/root-extractor.js

@@ -47,13 +47,15 @@ function findMatchingSelector($, selectors, extractHtml, allowMultiple) {
       }
 
       const [s, attr] = selector;
-      return (
-        (allowMultiple || (!allowMultiple && $(s).length === 1)) &&
-        $(s).attr(attr) &&
-        $(s)
-          .attr(attr)
-          .trim() !== ''
-      );
+      if (attr)
+        return (
+          (allowMultiple || (!allowMultiple && $(s).length === 1)) &&
+          $(s).attr(attr) &&
+          $(s)
+            .attr(attr)
+            .trim() !== ''
+        );
+      selector = s;
     }
 
     return (