Add support for captions in Reddit albums

Albums with captions:
- https://redd.it/zlu89r
- https://redd.it/tejlvr
- https://redd.it/teju6i
- https://redd.it/tepyv9

Album with only one caption: https://redd.it/zmhu48
Album with no caption: https://redd.it/t3urgf

redditbot.py

@@ -402,20 +402,26 @@ async def main():
                 elif getattr(random_post, 'is_gallery', None):
                     files = []
                     captions = []
-                    if getattr(random_post, 'gallery_data', None):
-                        gallery_keys = map(lambda i: i[1], sorted(map(lambda i: (i['id'], i['media_id']), random_post.gallery_data['items']), key=lambda i: i[0]))
-                    else:
-                        gallery_keys = random_post.media_metadata.keys()
-                    for a, i in enumerate(gallery_keys):
-                        i = random_post.media_metadata[i]
-                        if i['status'] == 'valid':
-                            filename = os.path.join(tempdir, str(time.time()))
-                            for j in ('u', 'mp4', 'gif'):
-                                if j in i['s']:
-                                    await _download_file(filename, i['s'][j])
-                                    break
-                            captions.append(f'{text}\n#{a + 1}')
-                            files.append(filename)
+                    gallery_data = sorted(random_post.gallery_data['items'], key=lambda i: i['id'])
+                    # no enumerate() because we want to update the count only if the media's status is valid
+                    count = 1
+                    for gallery_data_item in gallery_data:
+                        media_metadata_item = random_post.media_metadata[gallery_data_item['media_id']]
+                        if media_metadata_item['status'] != 'valid':
+                            continue
+                        filename = os.path.join(tempdir, str(time.time()))
+                        for media_type in ('u', 'mp4', 'gif'):
+                            if media_type not in media_metadata_item['s']:
+                                continue
+                            await _download_file(filename, media_metadata_item['s'][media_type])
+                            break
+                        caption = f'{text}\n#{count}'
+                        real_caption = (gallery_data_item.get('caption') or '').strip()
+                        if real_caption:
+                            caption += f': {html.escape(real_caption)}'
+                        captions.append(caption)
+                        files.append(filename)
+                        count += 1
                     url = None
                 if url:
                     parsed = list(urlparse(url))