[facebook] Support single-video ID links

I stumbled upon this at https://www.facebook.com/bwfbadminton/posts/10157127020046316 . No idea how prevalent it is yet.

youtube_dl/extractor/facebook.py

@@ -466,15 +466,18 @@ class FacebookIE(InfoExtractor):
             return info_dict
 
         if '/posts/' in url:
+            video_id_json = self._search_regex(
+                r'(["\'])video_ids\1\s*:\s*(?P<ids>\[.+?\])', webpage, 'video ids', group='ids',
+                default='')
+            if video_id_json:
                 entries = [
                     self.url_result('facebook:%s' % vid, FacebookIE.ie_key())
-                for vid in self._parse_json(
-                    self._search_regex(
-                        r'(["\'])video_ids\1\s*:\s*(?P<ids>\[.+?\])',
-                        webpage, 'video ids', group='ids'),
-                    video_id)]
-
+                    for vid in self._parse_json(video_id_json, video_id)]
                 return self.playlist_result(entries, video_id)
+
+            # Single Video?
+            video_id = self._search_regex(r'video_id:\s*"([0-9]+)"', webpage, 'single video id')
+            return self.url_result('facebook:%s' % video_id, FacebookIE.ie_key())
         else:
             _, info_dict = self._extract_from_url(
                 self._VIDEO_PAGE_TEMPLATE % video_id,