Allow send URL to be provided interactively

[ehuggett]

Accepting the send URL on the command line may compromise the secrecy of its secretKey

• CLI arguments can be read by other processes e.g. from /proc/$pid/cmdline (normally world readable).
• It may be saved to the shells history file e.g. .bash_history (normally only readable by the user that owns it).

To close this issue

• ~~If --url is specified without a value, prompt for it.~~
• a warning should be printed if a secret value is passed on the CLI indicating that this is a bad practice.
  • provide a more detailed description of the risks in the README