[enh] Add onions category with Ahmia, Not Evil and Torch

Xpath engine and results template changed to account for the fact that
archive.org doesn't cache .onions, though some onion engines migth have
their own cache.

Disabled by default. Can be enabled by setting the SOCKS proxies to
wherever Tor is listening and setting using_tor_proxy as True.

Requires Tor and updating packages.

To avoid manually adding the timeout on each engine, you can set
extra_proxy_timeout to account for Tor's (or whatever proxy used) extra
time.

searx/engines/__init__.py

@@ -142,6 +142,17 @@ def load_engine(engine_data):
         engine.stats['page_load_time'] = 0
         engine.stats['page_load_count'] = 0
 
+    # tor related settings
+    if settings['outgoing'].get('using_tor_proxy'):
+        # use onion url if using tor.
+        if hasattr(engine, 'onion_url'):
+            engine.search_url = engine.onion_url + getattr(engine, 'search_path', '')
+    elif 'onions' in engine.categories:
+        # exclude onion engines if not using tor.
+        return None
+
+    engine.timeout += settings['outgoing'].get('extra_proxy_timeout', 0)
+
     for category_name in engine.categories:
         categories.setdefault(category_name, []).append(engine)
 
@@ -252,8 +263,9 @@ def get_engines_stats(preferences):
 
 
 def load_engines(engine_list):
-    global engines
+    global engines, engine_shortcuts
     engines.clear()
+    engine_shortcuts.clear()
     for engine_data in engine_list:
         engine = load_engine(engine_data)
         if engine is not None: