[exhentai] implement 'tags' option (#2117)

allow splitting tags into categories,
e.g. 'tags_parody', 'tags_group', etc.

test/results/exhentai.py

@@ -13,7 +13,7 @@ __tests__ = (
     "#url"     : "https://exhentai.org/g/1200119/d55c44d3d0/",
     "#category": ("", "exhentai", "gallery"),
     "#class"   : exhentai.ExhentaiGalleryExtractor,
-    "#options"     : {"original": False},
+    "#options"     : {"original": False, "tags": True},
     "#sha1_content": [
         "2c68cff8a7ca540a78c36fdbf5fbae0260484f87",
         "e9891a4c017ed0bb734cd1efba5cd03f594d31ff",
@@ -40,6 +40,10 @@ __tests__ = (
         "group:seventh lowlife",
         "other:sample",
     ],
+    "tags_parody" : ["komi-san wa komyushou desu."],
+    "tags_character": ["shouko komi"],
+    "tags_group"  : ["seventh lowlife"],
+    "tags_other"  : ["sample"],
     "thumb"       : "https://s.exhentai.org/t/ce/0a/ce0a5bcb583229a9b07c0f83bcb1630ab1350640-624622-736-1036-jpg_250.jpg",
     "title"       : "C93 [Seventh_Lowlife] Komi-san ha Tokidoki Daitan desu (Komi-san wa Komyushou desu) [Sample]",
     "title_jpn"   : "(C93) [Comiketjack (わ！)] 古見さんは、時々大胆です。 (古見さんは、コミュ症です。) [見本]",