Prompt for password if it isn't passed in #15

It might be helpful for people who are paranoid about their bash history to have the option of entering their password interactively.

Additionally, it could detect if you passed an empty string and prompt for it in that case as well.

A workaround at the moment is to prefix your invocation of the command with a space which will cause bash (and maybe other shells?) to not log it in your history.

Fixed in https://github.com/weskerfoot/DeleteFB/pull/16