[event-participant] Improve control flow

[event-participant] Improve control flow

Print summary if no STDIN is provided, instead of if input was empty.
Create file if not existing. Create cache if not existing. Update help
accordingly.

TODO Provide arguments as ordinary flags: `-f` for file, `-n` for
"other name", `-c` for cancel, argument can be "comment".

TODO Provide long list option for printing: `-s` for summary.

TODO Provide title row and header row for the CSV-file: `-t` for
setting title.

TODO Provide PGP encryption by default: `-r` for GPG recipiant.