[wallhaven] rewrite

- use API
- remove login support, add 'api-key' option
- remove support for "alpha" subdomain - alpha.wallhaven.cc used numeric
  IDs that can't be translated to the new ID system
- support direct links to wallpapers

docs/gallery-dl.conf

@@ -134,6 +134,10 @@
             "retweets": true,
             "videos": false
         },
+        "wallhaven":
+        {
+            "api-key": null
+        },
         "booru":
         {
             "tags": false