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.

docs/gallery-dl.conf

@@ -1,5 +1,6 @@
 {
     "base-directory": "/tmp/",
+    "netrc": false,
     "downloader":
     {
         "http":