[danbooru] add authentication support (closes #151)

... via HTTP Basic Auth with username and "password".

The password value in this case is not the account password itself,
but the"api_key" found in your user profile.

docs/configuration.rst

@@ -148,12 +148,15 @@ Description The username and password to use when attempting to log in to
 
             Specifying username and password is
             required for the ``pixiv``, ``nijie`` and ``seiga`` modules and
-            optional (but strongly recommended) for ``exhentai``,
+            optional (but strongly recommended) for ``danbooru``, ``exhentai``,
             ``sankaku`` and ``idolcomplex``.
 
             These values can also be set via the ``-u/--username`` and
             ``-p/--password`` command-line options or by using a |.netrc|_ file.
             (see Authentication_)
+
+            Note: The password for ``danbooru`` is the API key found in your
+            user profile, not the password for your account.
 =========== =====