Allow modification of 'incomplete-dir' location

[klogg]

Sometimes it is necessary to change not only location of 'destination' but also location of 'incomplete-dir' where temporary downloads are stored as configured by Transmissions 'resume' files.

Added 2 options: -d or --destination to change torrent's destination location -i or --incomplete-dir to change torrents incomplete location Now it is mandantory to specify one of them when running the script. The logic was tested and checked with shellcheck.

README and AUTHORS are updated accordingly.

[vlevit]

Thank you Artem! Please fix misspelled parameter and I will merge it.