[ngomik] add replacement for 'subapics'

http://subapics.com/ got discontinued and replaced by http://ngomik.in/.

ngomik.in is still displaying a link to the "old site" showing a big
"Account Suspended" sign.

gallery_dl/extractor/__init__.py

@@ -58,6 +58,7 @@ modules = [
     "mangareader",
     "mangastream",
     "myportfolio",
+    "ngomik",
     "nhentai",
     "nijie",
     "nyafuu",
@@ -80,7 +81,6 @@ modules = [
     "simplyhentai",
     "slideshare",
     "smugmug",
-    "subapics",
     "thebarchive",
     "tumblr",
     "twitter",