Save stacks using an alias, allowing for `c66 deploy -s production`

I tested the code a bit, but no guarantees it doesn't break anything. I would have saved the aliases in the stack.json file, but I didn't want to break previous versions of the gem. (Saving multiple aliases would require a serialized Array in stack.json, not a Hash, and I didn't want to deal with that check.)

Let me know what you think.