Ability to not overwrite generated files

[Etomyutikos]

As per the Slack discussion:

• Our use case is this: we want Gnorm to generate some files once; those files will then be manually edited.
• Some files Gnorm should always generate. Some, it should only generate if they don't already exist.
• it’s easy enough to say that if an output file matches a glob pattern (like "*.perm.go" or something), then don’t overwrite it.
• probably a list of paths to run through a glob parser is fine. NoOverwrite = ["*.perm.go", "*.xml"]