--autofill functionality for passwords on SSH connections (rewritten)

This implements the same functionality as my previous merge request, but is completely rewritten to implement some of the suggestions on the aforementioned PR based upon the dev branch. Getpass is now used instead of plaintext passwords in the command, allowing Clickable to optionally prompt once and to fill out subsequent prompts automatically. I've also changed the --password flag to --autopass, which seems to be more descriptive of its functionality (perhaps we can discuss if there are any other suggestions?)

No breaking changes are present on existing workflows. Clickable checks to see if sshpass is present on the host system before attempting to utilize this functionality (now uses is_command() properly), and will simply gracefully ignore --autopass if the required dependencies aren't present.

related to #369