ssh-commander v0.3

Most prominent changes, since v0.2 release:

• Add SSH key-based authentication support!.
• Validate remote hosts against the known_hosts file!.
• Minor improvement at session handling.
• Fail gracefully when no valid remote hosts are found!.
• Set default timeout value for sessions at 5 seconds!.
• Require Python v3.7 or newer!.
• Update documentation at the README file.