Upgrade github.com/urfave/cli to v2 (#551)

`urfave/cli` v2 was released in 2019, and it doesn't introduce relevant
user-facing breaking changes.

Migration guide from v1 can be found at: https://cli.urfave.org/migrate-v1-to-v2/



The only user-facing change is within the "Flags before args" section,
but I haven't found that to affect `transfer.sh` as arguments are not
used.

Co-authored-by: Stefan Benten <mail@stefan-benten.de>
Co-authored-by: Andrea Spacca <andrea.spacca@gmail.com>