Add key to settings.yml for opening result links in a new tab

Closes #1552
Closes #444

searx/preferences.py

@@ -364,7 +364,7 @@ class Preferences(object):
                 choices=themes
             ),
             'results_on_new_tab': MapSetting(
-                False,
+                settings['ui'].get('results_on_new_tab', False),
                 map={
                     '0': False,
                     '1': True,

searx/settings.yml

@@ -25,6 +25,7 @@ ui:
     default_locale : "" # Default interface locale - leave blank to detect from browser information or use codes from the 'locales' config section
     theme_args :
         oscar_style : logicodev # default style of oscar
+#   results_on_new_tab: False  # Open result links in a new tab by default
 #   categories_order :
 #     - general
 #     - files