Output diff feature

[jonvmey]

I think that having an output diff/dry-run option would be a useful addition. Basically you could do a dry-run of the templater and display a diff between the output of the current rules and source files and the existing contents of the destination files (if they exist) without actually overwriting the destination files. This would be useful for verifying that existing files won't accidentally be blown away by running the templater as well as detecting any local modifications in the destination files that should be back-annotated into the rules and source files.

I'd be happy to work on implementing this or some variation of it if you think it's a good idea.