Added history expansion

See issue #7 .
http://tiswww.case.edu/php/chet/readline/history.html#SEC16 mentions "If an error occurred in expansion, then output contains a descriptive error message."
--> don't print `errno` - which is "Success" even if an error (eg !!:0::::::) occurs - , but `expansion` (which seems to be empty however) and then `free()` it