add digbt engine

Unfortunately, it is quite slow so it is disabled.
Furthermore, the display of number of files is wrong
on digbt.org, so it is not displayed on searx.

searx/settings.yml

@@ -87,7 +87,7 @@ engines:
   - name : btdigg
     engine : btdigg
     shortcut : bt
-    
+
   - name : crossref
     engine : json_engine
     paging : True
@@ -118,6 +118,11 @@ engines:
     weight : 2
     disabled : True
 
+  - name : digbt
+    engine : digbt
+    shortcut : dbt
+    timeout : 6.0
+
   - name : digg
     engine : digg
     shortcut : dg