implement 'chapter-reverse' option (#149)

Setting it to `true` will start with the latest chapter instead of the
first one.

docs/configuration.rst

@@ -797,6 +797,18 @@ Description Categorize tags by their respective types
 =========== =====
 
 
+extractor.[manga-extractor].chapter-reverse
+-------------------------------------------
+=========== =====
+Type        ``bool``
+Default     ``false``
+Description Reverse the order of chapter URLs extracted from manga pages.
+
+            * `true`: Start with the latest chapter
+            * `false`: Start with the first chapter
+=========== =====
+
+
 
 Downloader Options
 ==================

gallery_dl/extractor/common.py

@@ -248,6 +248,9 @@ class MangaExtractor(Extractor):
         Extractor.__init__(self)
         self.url = url or self.scheme + "://" + match.group(1)
 
+        if self.config("chapter-reverse", False):
+            self.reverse = not self.reverse
+
     def items(self):
         page = self.request(self.url).text