Allow missing appauthor to fall back to appname.

This change provides a default value for `appauthor` when on Windows.

### Rationale

#### 1. For DRY

It is often the case where the "author" refers to the group that works on a project. Like with GitHub organizations, it is also often that the name of the group reuses the name of the project. Instead of repeating the "appname" in the arguments, this change makes this automatic.

#### 2. For portability

Mac and Unix developers probably won't think about providing `appauthor` since it isn't used and isn't required. Meanwhile, a **user** running Windows will see an error. This fallback allows non-Windows developers to continue to ignore the unused argument, while enabling their project to work on Windows without any extra effort or waiting for a bug to be filed.