[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/__init__.py

@@ -38,6 +38,7 @@ def check_settings_yml(file_name):
     else:
         return None
 
+
 # find location of settings.yml
 if 'SEARX_SETTINGS_PATH' in environ:
     # if possible set path to settings using the