[enh] introduce private engines

This PR adds a new setting to engines named `tokens`.
It expects a list of tokens which lets searx validate
if the request should be accepted or not.

searx/engines/dummy-offline.py

@@ -0,0 +1,12 @@
+"""
+ Dummy Offline
+
+ @results     one result
+ @stable      yes
+"""
+
+
+def search(query, request_params):
+    return [{
+        'result': 'this is what you get',
+    }]