[furaffinity] Add 'thumbnail' (#1284) and 'folders' properties

Retrieve 'thumbnail' and 'folders' properties for each post.
'thumbnail' (#1284):
 - Preview image used for search results, writing posts, music, etc.
 - Filename format: <post_id>@600-<directory_containing_full_image>.jpg
'Folders' (related to #1817):
 - A list of all gallery folders containing this post
 - Folder name format: [<folder_category> - ]<folder_name>
 - Only works on new layout; old layout does not show folders, so list will be empty

A test is included for each property.

test/results/furaffinity.py

@@ -115,6 +115,50 @@ __tests__ = (
     "width"      : 1280,
 },
 
+{
+    "#url"     : "https://www.furaffinity.net/view/22964019/",
+    "#comment" : "get thumbnails for posts (#1284)",
+    "#category": ("", "furaffinity", "post"),
+    "#class"   : furaffinity.FuraffinityPostExtractor,
+
+    "artist"      : "Dwale",
+    "artist_url"  : "dwale",
+    "date"        : "dt:2017-03-21 14:21:29",
+    "fa_category" : "Poetry",
+    "filename"    : "1490106089.dwale_poem_for_children",
+    "folders"     : [],
+    "height"      : 50,
+    "id"          : 22964019,
+    "rating"      : "General",
+    "title"       : "Poem for Children Wishing to Summon Evil Spirits",
+    "thumbnail"   : "https://t.furaffinity.net/22964019@600-1490106089.jpg",
+    "width"       : 50,
+},
+
+{
+    "#url"     : "https://www.furaffinity.net/view/34260156/",
+    "#comment" : "list gallery folders for image",
+    "#category": ("", "furaffinity", "post"),
+    "#class"   : furaffinity.FuraffinityPostExtractor,
+
+    "artist"      : "dbd",
+    "artist_url"  : "dbd",
+    "date"        : "dt:2019-12-17 22:52:01",
+    "fa_category" : "All",
+    "filename"    : "1576623121.dbd_patreoncustom-wdg13-web",
+    "folders"     : ["By Year - 2019",
+                     "Custom Character Folder - All Custom Characters",
+                     "Custom Character Folder - Other Ungulates",
+                     "Custom Character Folder - Female",
+                     "Custom Character Folder - Patreon Supported Custom Characters"],
+    "height"      : 900,
+    "id"          : 34260156,
+    "rating"      : "General",
+    "title"       : "Patreon Custom Deer",
+    "thumbnail"   : "https://t.furaffinity.net/34260156@600-1576623121.jpg",
+    "width"       : 488,
+},
+
 {
     "#url"     : "https://furaffinity.net/view/21835115/",
     "#category": ("", "furaffinity", "post"),