Handle no uid/gid supplied

related to #115 This handles logic for when a uid isn't specified on an existing destination file. The goal is to preserve existing uid/gid on destinations when not specified in the config.
Or we can do it in one step:

```
tc := &TemplateResourceConfig{
    TemplateResource: TemplateResource{Uid: -1, Gid: -1},
}
```