Add the ability to require tty on a per user basis

[Moncky]

I have a reuqirement that in general not to allow sudo without a tty except on a small number of system users.

I have submitted pull request #11 which fixes that.

Im not sure if having the same variable name globally and per user is how you'd prefer to do it, It makes sense to me to do it this way. let me know if you disagree and I can fix it up.

[rendhalver]

That is a much better way to do it.
Thanks.

[rendhalver]

I will give that a test later today and let you know how it goes.