Add encryption support

[andrewvaughan]

Allow encryption, if Passlib is installed, to take place when storing the variable. Add the following options:

• encryption is one of the options from Passlib
• salt the salt to use
• salt_size is the size of the salt to generate

Throw a warning if both salt and salt_size are set.

[BarbzYHOOL]

So actually we can't use the prompt for passwords?