add prompt password option

[kamiraux]

I added an option to allow prompting for the password instead of having it stored in the config file or put it on the command line. It is way better for security concerns.

It still supports previous behavior without any change.

To use this option:
transmission-remote-cli -c user@host --prompt-password
or
transmission-remote-cli -c user@host --prompt-password --create-config

[fagga]

It would be much nicer if it could detect if a password is needed and ask if it's not stored in the config file. What do you think?

[kamiraux]

Detect if a password is needed? You mean if the password is empty? Or try to connect and ask for the password if ever it fails?
I can probably do this quite easily.

[fagga]

I mean this: try to connect and ask for the password if ever it fails

Automation is always a good thing. If you can dynamically request the password instead of failing and having the user dig for the correct option to ask for it, I think the best path is obvious.

[jakem72360]

@fagga I managed to get it to prompt for a password automatically if one isn't provided, but required. Furthermore, the user can also be prompted for a username, if one wasn't provided in the connect string. I've started a separate pull request for these changes (#177).