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)

searx/plugins/self_useragent.py

@@ -0,0 +1,36 @@
+'''
+searx is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+searx is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License
+along with searx. If not, see < http://www.gnu.org/licenses/ >.
+
+(C) 2015 by Adam Tauber, <asciimoo@gmail.com>
+'''
+from flask.ext.babel import gettext
+import re
+name = "Self User Agent"
+description = gettext('Display your own User Agent if the query expression contains "user agent" or "user-agent"')
+default_on = True
+
+
+# User Agent query regex
+p = re.compile('user[ -]agent', re.IGNORECASE)
+
+
+# attach callback to the post search hook
+#  request: flask request object
+#  ctx: the whole local context of the pre search hook
+def post_search(request, ctx):
+    if p.match(ctx['search'].query):
+        ua = request.user_agent
+        ctx['search'].answers.clear()
+        ctx['search'].answers.add(ua)
+    return True