[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/settings.yml

@@ -60,8 +60,10 @@ outgoing: # communication with search engines
 # see http://docs.python-requests.org/en/latest/user/advanced/#proxies
 # SOCKS proxies are also supported: see http://requests.readthedocs.io/en/master/user/advanced/#socks
 #    proxies :
-#        http : http://127.0.0.1:8080
-#        https: http://127.0.0.1:8080
+#        http : socks5h://127.0.0.1:9050
+#        https: socks5h://127.0.0.1:9050
+#    using_tor_proxy : True
+#    extra_proxy_timeout : 10.0 # Extra seconds to add in order to account for the time taken by the proxy
 # uncomment below section only if you have more than one network interface
 # which can be the source of outgoing search requests
 #    source_ips:
@@ -89,6 +91,12 @@ engines:
     shortcut: apkm
     disabled: True
 
+# Requires Tor
+  - name : ahmia
+    engine : ahmia
+    categories : onions
+    shortcut : ah
+
   - name : arch linux wiki
     engine : archlinux
     shortcut : al
@@ -185,7 +193,7 @@ engines:
   - name : deviantart
     engine : deviantart
     shortcut : da
-    timeout: 3.0
+    timeout : 3.0
 
   - name : ddg definitions
     engine : duckduckgo_definitions
@@ -514,6 +522,11 @@ engines:
     timeout: 5.0
     shortcut : npm
 
+# Requires Tor
+  - name : not evil
+    engine : not_evil
+    shortcut : ne
+
   - name : nyaa
     engine : nyaa
     shortcut : nt
@@ -698,6 +711,18 @@ engines:
     url: https://torrentz2.eu/
     timeout : 3.0
 
+# Requires Tor
+  - name : torch
+    engine : xpath
+    paging : True
+    search_url : http://xmh57jrknzkhv6y3ls3ubitzfqnkrwxhopf5aygthi7d6rplyvk3noyd.onion/cgi-bin/omega/omega?P={query}&DEFAULTOP=and
+    results_xpath : //table//tr
+    url_xpath : ./td[2]/a
+    title_xpath : ./td[2]/b
+    content_xpath : ./td[2]/small
+    categories : onions
+    shortcut : tch
+
   - name : twitter
     engine : twitter
     shortcut : tw