Allow value tty for system property picocli.ansi

[remkop]

Currently users can force picocli to emit ANSI escape characters by setting system property picocli.ansi to true (or false to never emit ANSI escape chars).

This does not take into account whether the end user is using an interactive console.

If the application output is redirected, the application should not emit ANSI escape characters.

This ticket proposes that picocli should interpret a system property picocli.ansi value of tty to mean: emit ANSI escape characters if on a TTY (interactive console).