Add a self user agent plugin

Just like with the "ip" query, duckduckgo gives the server's
information with the "user agent" query.
This corrects this behavior by adding a plugin based on self_ip.py plugin.

searx/plugins/__init__.py

@@ -21,6 +21,7 @@ logger = logger.getChild('plugins')
 
 from searx.plugins import (https_rewrite,
                            self_ip,
+                           self_useragent,
                            search_on_category_select)
 
 required_attrs = (('name', str),
@@ -72,4 +73,5 @@ class PluginStore():
 plugins = PluginStore()
 plugins.register(https_rewrite)
 plugins.register(self_ip)
+plugins.register(self_useragent)
 plugins.register(search_on_category_select)