Escape HTML characters when printing the train

Since it's wrapped in a `<pre>`, I assume it's meant to be inserted into
HTML?

tilde-train.py

@@ -41,6 +41,7 @@ import curses
 from signal import signal, SIGINT
 import time # allowing the loop steps of train animation to be slowed
 import string # for input validation
+from html import escape
 from inspect import cleandoc
 from pathlib import Path
 
@@ -339,7 +340,7 @@ train_str = "\n".join(train)
 
 if print_train:
   print("<pre>")
-  print(train_str)
+  print(escape(train_str))
   print("</pre>")
   sys.exit()