[keenspot] add comic extractor (#223)

Doesn't work for
- http://brawlinthefamily.keenspot.com/
- http://flipside.keenspot.com/
- http://lastblood.keenspot.com/
- http://mysticrevolution.keenspot.com/
- http://porcelain.keenspot.com/
- http://twokinds.keenspot.com/
yet, because of custom layouts.

gallery_dl/extractor/__init__.py

@@ -44,6 +44,7 @@ modules = [
     "imgth",
     "imgur",
     "instagram",
+    "keenspot",
     "khinsider",
     "kissmanga",
     "komikcast",