Avoid beeping when setting the console title if non-ascii chars in $PWD.

Fix for issue #128 where the prompt causes the console to beep if there are
non-ASCII characters in the path. This patch replaces all non-ASCII characters
with a question mark when expanding the prompt

Signed-off-by: Anselm Kruis <a.kruis@science-computing.de>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>