[mod] Archlinux Wiki: improved request API & upgrade to data_type: traits_v1

re-implementation of the Archlinux Wiki:

- fetch_traits(): fetch languages, wiki URLs and title arguments
- add content field to the result list
- add documentation

Wikis from wiki.archlinux.fr, wiki.archlinux.ro, archtr.org/wiki do no longer
exists (has been merged in the main wiki).

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/sxng_locales.py

@@ -81,6 +81,7 @@ sxng_locales = (
     ('ru-RU', 'Русский', 'Россия', 'Russian', '\U0001f1f7\U0001f1fa'),
     ('sk', 'Slovenčina', '', 'Slovak', '\U0001f310'),
     ('sl', 'Slovenščina', '', 'Slovenian', '\U0001f310'),
+    ('sr', 'Српски', '', 'Serbian', '\U0001f310'),
     ('sv', 'Svenska', '', 'Swedish', '\U0001f310'),
     ('sv-SE', 'Svenska', 'Sverige', 'Swedish', '\U0001f1f8\U0001f1ea'),
     ('th', 'ไทย', '', 'Thai', '\U0001f310'),