Enable unix socket to work with token plugin

This commit adds support to unix sockets in the token plugin, thus it is
possible to have a token files like:

  token: unix_socket:/path/to/socket_file

A single websockify instance will be able to handle multiple sockets.

Signed-off-by: Jose Ricardo Ziviani <jose@ziviani.net>

tests/test_websocketproxy.py

@@ -93,6 +93,16 @@ class ProxyRequestHandlerTestCase(unittest.TestCase):
         self.assertEqual(host, "some host")
         self.assertEqual(port, "some port")
 
+    def test_get_target_unix_socket(self):
+        class TestPlugin(token_plugins.BasePlugin):
+            def lookup(self, token):
+                return ("unix_socket", "/tmp/socket")
+
+        _, socket = self.handler.get_target(
+            TestPlugin(None), self.handler.path)
+
+        self.assertEqual(socket, "/tmp/socket")
+
     def test_get_target_raises_error_on_unknown_token(self):
         class TestPlugin(token_plugins.BasePlugin):
             def lookup(self, token):