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/configuration.rst

@@ -23,6 +23,15 @@ Description Directory path used as the base for all download destinations.
 =========== =====
 
 
+netrc
+-----
+=========== =====
+Type        ``bool``
+Default     ``false``
+Description Enable the use of |.netrc|_ authentication data.
+=========== =====
+
+
 cache.file
 ----------
 =========== =====
@@ -331,7 +340,7 @@ Description The value of the ``limit`` parameter when loading
 
 
 extractor.reddit.morecomments
--------------------------
+-----------------------------
 =========== =====
 Type        ``bool``
 Default     ``false``
@@ -392,11 +401,13 @@ Description The ``refresh_token`` value you get from linking your Reddit account
 =========== =====
 
 
+.. |.netrc| replace:: ``.netrc``
 .. |tempfile.gettempdir()| replace:: ``tempfile.gettempdir()``
 .. |requests.get()| replace:: ``requests.get()``
 .. |mature_content| replace:: ``mature_content``
 .. |webbrowser.open()| replace:: ``webbrowser.open()``
 
+.. _.netrc:            https://stackoverflow.com/tags/.netrc/info
 .. _tempfile.gettempdir(): https://docs.python.org/3/library/tempfile.html#tempfile.gettempdir
 .. _requests.get():    https://docs.python-requests.org/en/latest/user/advanced/#timeouts
 .. _format string:     https://docs.python.org/3/library/string.html#formatstrings

docs/gallery-dl.conf

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