[enh] add settings option to set listening address - closes #397

searx/settings.yml

 server:
     port : 8888
+    bind_address : "127.0.0.1" # address to listen on
     secret_key : "ultrasecretkey" # change this!
     debug : False # Debug mode, only for development
     request_timeout : 2.0 # seconds

searx/settings_robot.yml

 server:
     port : 11111
+    bind_address : 127.0.0.1
     secret_key : "ultrasecretkey" # change this!
     debug : False
     request_timeout : 3.0 # seconds

searx/webapp.py

     app.run(
         debug=settings['server']['debug'],
         use_debugger=settings['server']['debug'],
-        port=settings['server']['port']
+        port=settings['server']['port'],
+        host=settings['server']['bind_address']
     )