[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/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"]