add generalized extractors for Mastodon instances (#144)

Extractors for Mastodon instances can now be dynamically generated,
based on the instance names in the 'extractor.mastodon.*' config path.

Example:
{
    "extractor": {
        "mastodon": {
            "pawoo.net": { ... },
            "mastodon.xyz": { ... },
            "tabletop.social": { ... },
            ...
        }
    }
}

Each entry requires an 'access-token' value, which can be generated with
'gallery-dl oauth:mastodon:<instance URL>'.
An 'access-token' (as well as a 'client-id' and 'client-secret') for
pawoo.net is always available, but can be overwritten as necessary.

gallery_dl/extractor/__init__.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2015-2018 Mike Fährmann
+# Copyright 2015-2019 Mike Fährmann
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 as
@@ -67,7 +67,6 @@ modules = [
     "nijie",
     "nyafuu",
     "paheal",
-    "pawoo",
     "piczel",
     "pinterest",
     "pixiv",
@@ -95,6 +94,7 @@ modules = [
     "yandere",
     "xvideos",
     "yuki",
+    "mastodon",
     "imagehosts",
     "directlink",
     "recursive",