NQ41 RPN Calculator

[Egan Ford]

Feature requests:

1. ctrl-d to exit from prompt, common for UNIX/Linux CLI tool prompts
2. args to CLI, e.g. NQ-41 3 4 +, returns 7, no interactive prompt
3. support stdin, e.g. echo "3 4 +" | NQ-41, returns 7, no interactive prompt