combine imagehost extractors into a single file

added extractors for
 - hosturimage.com
 - imageontime.org
 - imgupload.yt
 - imgspice.com
 - imgclick.net

All of these would have shared a lot of common code, so i created a base
class for imagehost extractors and put them all in the same file to
avoid clutter.

gallery_dl/extractor/__init__.py

@@ -41,7 +41,6 @@ modules = [
     "imgth",
     "imgtrex",
     "imgur",
-    "imgyt",
     "khinsider",
     "kissmanga",
     "konachan",
@@ -58,7 +57,6 @@ modules = [
     "picmaniac",
     "pinterest",
     "powermanga",
-    "rapidimg",
     "rule34",
     "safebooru",
     "sankaku",
@@ -72,6 +70,7 @@ modules = [
     "whentai",
     "worldthree",
     "yandere",
+    "imagehosts",
     "recursive",
 ]