Prompt user for string if can_encode.py is invoked without an argument

This allows usage on Windows, which by default does not support unicode
fully in the command line.

can_encode.py

@@ -2,7 +2,10 @@ import sys
 
 import char_encodings
 
-decoded = sys.argv[1]
+if len(sys.argv) == 2:
+	decoded = sys.argv[1]
+else:
+	decoded = input()
 
 singlebyte = {}
 multibyte = {}