[tumblr] add options to control extraction behavior (#48)

- posts   : list of post-types to inspect
- inline  : scan post bodies for inline images
- external: follow external links

docs/gallery-dl.conf

@@ -100,6 +100,12 @@
         {
             "mp4": true
         },
+        "tumblr":
+        {
+            "posts": "photo",
+            "inline": false,
+            "external": false
+        },
         "recursive":
         {
             "blacklist": ["directlink", "oauth", "recursive", "test"]