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.

test/test_extractor.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 
-# Copyright 2018 Mike Fährmann
+# Copyright 2018-2019 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
@@ -149,6 +149,8 @@ class TestExtractor(unittest.TestCase):
         def capitalize(c):
             if "-" in c:
                 return string.capwords(c.replace("-", " ")).replace(" ", "")
+            if "." in c:
+                c = c.replace(".", "")
             return c.capitalize()
 
         mapping = {