[mod] Make a default domain for ebay

Which can be changed.

searx/settings.yml

@@ -514,6 +514,7 @@ engines:
   # - name: ebay
   #   engine: ebay
   #   shortcut: eb
+  #   base_url: 'https://www.ebay.com'
   #   disabled: true
   #   timeout: 5