add .netrc support (#22)

Use the '--netrc' cmdline option or set the 'netrc' config option
to 'true' to enable the use of .netrc authentication data.

The 'machine' names for the .netrc info are the lowercase extractor
names (or categories): batoto, exhentai, nijie, pixiv, seiga.

gallery_dl/extractor/common.py

@@ -10,6 +10,7 @@
 
 import os
 import time
+import netrc
 import queue
 import logging
 import requests
@@ -42,6 +43,24 @@ class Extractor():
         return config.interpolate(
             ("extractor", self.category, self.subcategory, key), default)
 
+    def auth_info(self):
+        """Return authentication information as (username, password) tuple"""
+        username = self.config("username")
+        password = None
+
+        if username:
+            password = self.config("password")
+        elif config.get(("netrc",), False):
+            try:
+                info = netrc.netrc().authenticators(self.category)
+                username, _, password = info
+            except (OSError, netrc.NetrcParseError) as exc:
+                self.log.error("netrc: %s", exc)
+            except TypeError:
+                self.log.warning("netrc: No authentication info")
+
+        return username, password
+
     def request(self, url, encoding=None, *args, **kwargs):
         response = safe_request(self.session, url, *args, **kwargs)
         if encoding: