feat(cli): add delimiter option for sub file write

[ysfscream]

PR Checklist

What is the current behavior?

Users cannot change the delimiter for writing to a file because the default value is only the '\n.'

What is the new behavior?

{"temp":59,"hum":86}
{"temp":71,"hum":75}
{"temp":36,"hum":40}
{"temp":53,"hum":49}

mqttx bench pub -c 1 -h 127.0.0.1 -t 'smart_home' --file-read ./data-split-newline.txt --split -im 1000

mqttx sub -h 127.0.0.1 -p 1883 -t smart_home --file-write ./delimiter-file.txt --delimiter ','

{"temp":59,"hum":86},{"temp":71,"hum":75},{"temp":36,"hum":40},{"temp":53,"hum":49},