[enh] reduce the available language list

close #406

settings['search']['languages'] contains the languages codes and 'all'

searx/settings.yml

@@ -20,6 +20,13 @@ search:
   # Default search language - leave blank to detect from browser information or
   # use codes from 'languages.py'
   default_lang: ""
+  # Available languages
+  # languages:
+  #  - all
+  #  - es
+  #  - de
+  #  - it-IT
+  #  - en-GB
   # ban time in seconds after engine errors
   ban_time_on_fail: 5
   # max ban time in seconds after engine errors