Add an option to warn on missing variables

[Cellivar]

As a deployment administrator I would like to know when a release definition is missing a variable called out in a template file because sometimes variables are added but missing in a release.

Similar tools implement this functionality but do not implement running within archives. It would be very useful to throw warnings when variables are called out but missing.

Consider adding an option for throwing errors (and breaking the build) as well.

Thanks!

[melborp]

Good idea. I could also add extra verbosity.