[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.
 =========== =====
 
 

docs/gallery-dl.conf

@@ -14,6 +14,11 @@
         {
             "external": false
         },
+        "danbooru":
+        {
+            "username": null,
+            "password": null
+        },
         "deviantart":
         {
             "refresh-token": null,
@@ -72,6 +77,11 @@
             "password": null,
             "ugoira": true
         },
+        "reactor":
+        {
+            "wait-min": 3.0,
+            "wait-max": 6.0
+        },
         "recursive":
         {
             "blacklist": ["directlink", "oauth", "recursive", "test"]