Provides debuging option for TCP stream

The TCP BaseClient class has been modified to recognize an environment
variable 'PYMODES_DEBUG'. When the variable is set to 'true', the
stream will halt execution rather than restarting. This is for
debugging purposes and does not alter the main functionality of
the program.

pyModeS/extra/tcpclient.py

@@ -269,6 +269,15 @@ class BaseClient(Thread):
 
                 time.sleep(0.001)
             except Exception as e:
+
+                # Provides the user an option to supply the environment 
+                # variable PYMODES_DEBUG to halt the execution
+                # for debugging purposes
+                debug_intent = os.environ.get('PYMODES_DEBUG', None)
+                if debug_intent.lower() == 'true':
+                    sys.exc_info()
+                    sys.exit()
+                else:
                     print("Unexpected Error:", e)
 
                 try: