option to append to files in command-line tools

Joachim Klein has a scenario where he would like ltlcross --save-bogus=FILENAME to append to FILENAME instead of overwriting it.

Appending would also make sense for ltlcross --grind and for the planned --output option (see #56 (closed)). Maybe also for --csv (in this case we would not output the header).

Maybe we can have the convention that if FILENAME starts with >> it means we want to append to the file.