[enh] Allow passing headers/cookies from settings.yml

Example:

   - engine: xpath
   - search_url: example.org
   - headers: {'example_header': 'example_header'}
   - cookies: {'safesearch': 'off'}

searx/engines/xpath.py

@@ -85,6 +85,11 @@ suggestion_xpath = ''
 cached_xpath = ''
 cached_url = ''
 
+cookies = {}
+headers = {}
+'''Some engines might offer different result based on cookies or headers.
+Possible use-case: To set safesearch cookie or header to moderate.'''
+
 paging = False
 '''Engine supports paging [True or False].'''
 
@@ -166,6 +171,9 @@ def request(query, params):
         'safe_search': safe_search,
     }
 
+    params['cookies'] = cookies
+    params['headers'] = headers
+    
     params['url'] = search_url.format(**fargs)
     params['soft_max_redirects'] = soft_max_redirects