rework the '-g' cmdline option

the amount of how often the -g option is given now determines up until
what level URLs are resolved.

example:

$ gallery-dl -g http://kissmanga.com/Manga/Dropout
http://kissmanga.com/Manga/Dropout/Ch-000---Oneshot-?id=145847

- when applied to a manga-extractor, specifying the -g option once will
  now print a list of all chapter URls

$ gallery-dl -gg http://kissmanga.com/Manga/Dropout
http://2.bp.blogspot.com/.../000.png
http://2.bp.blogspot.com/.../001.png
...

- specifying it twice (or even more often) will go a level deeper and
  print the image URLs found in those chapters

gallery_dl/__init__.py

@@ -30,7 +30,7 @@ def build_cmdline_parser():
     parser = argparse.ArgumentParser(
         description='Download images from various sources')
     parser.add_argument(
-        "-g", "--get-urls", dest="list_urls", action="store_true",
+        "-g", "--get-urls", dest="list_urls", action="count",
         help="print download urls",
     )
     parser.add_argument(
@@ -141,6 +141,7 @@ def main():
 
             if args.list_urls:
                 jobtype = job.UrlJob
+                jobtype.maxdepth = args.list_urls
             elif args.list_keywords:
                 jobtype = job.KeywordJob
             else:

gallery_dl/job.py

@@ -153,13 +153,20 @@ class KeywordJob(Job):
 
 class UrlJob(Job):
     """Print download urls"""
+    maxdepth = -1
+
+    def __init__(self, url, depth=1):
+        Job.__init__(self, url)
+        self.depth = depth
+        if depth == self.maxdepth:
+            self.handle_queue = print
 
     def handle_url(self, url, _):
         print(url)
 
     def handle_queue(self, url):
         try:
-            UrlJob(url).run()
+            UrlJob(url, self.depth + 1).run()
         except exception.NoExtractorError:
             pass