[seiga] support more than 200 images

Due to API restrictions and/or missing knowledge about and
documentation of API usage, it was only possible to retrieve the
latest 200 images of a niconico seiga user with said API.

The new approach manually visits each HTML page and gets its
information from there.

test/test_extractors.py

@@ -83,6 +83,7 @@ skip = [
     "archivedmoe", "archiveofsins", "thebarchive",
     # temporary issues
     "mangazuki",
+    "hentaifoundry",  # invalid SSL cert
 ]
 # enable selective testing for direct calls
 if __name__ == '__main__' and len(sys.argv) > 1: