[webtoons] Add a new extractor for webtoons.com (#761)

The webtoons extractor can extract episode and entire comic (all
episodes) from webtoons.com.

All the logic of the extractors should be trivial except for a couple
of kludges needed:

 - `ageGatePass' cookie is always set to avoid possible redirect and stop of
    extraction, especially in the comic extractor
 - The image URLs returned by the episode extractor could not be fetched
   directly and the `Referer:' HTTP header needs to be passed to fetch them

Close #593.

scripts/supportedsites.py

@@ -86,6 +86,7 @@ CATEGORY_MAP = {
     "thebarchive"    : "The /b/ Archive",
     "vanillarock"    : "もえぴりあ",
     "vsco"           : "VSCO",
+    "webtoons"       : "Webtoon",
     "wikiart"        : "WikiArt.org",
     "worldthree"     : "World Three",
     "xhamster"       : "xHamster",