[gelbooru] re-enable API use (closes #56)

Gelbooru's API allows access to all images and is not restricted
to the first 20000.

This also adds an option to select between API use and manual
information extraction in case their API gets disabled again.

docs/configuration.rst

@@ -441,6 +441,18 @@ Description Sets the maximum allowed size for downloaded images.
 =========== =====
 
 
+extractor.gelbooru.api
+----------------------
+=========== =====
+Type        ``bool``
+Default     ``true``
+Description Enable use of Gelbooru's API.
+
+            Set this value to `false` if the API has been disabled to switch
+            to manual information extraction.
+=========== =====
+
+
 extractor.gfycat.format
 -----------------------
 =========== =====

docs/gallery-dl.conf

@@ -70,7 +70,8 @@
         },
         "gelbooru":
         {
-            "filename": "{category}_{id:>07}_{md5}.{extension}"
+            "filename": "{category}_{id:>07}_{md5}.{extension}",
+            "api": true
         },
         "reddit":
         {