Set the codepage to 65001 before generating

When I run `rake generate` on Windows, I get the following exception:

```
Liquid Exception: incompatible character encodings: UTF-8 and IBM437...
```

Turns out the fix is easy, I just need to set the code-page before running the generate command.

I added a check to ensure that the script is running on Windows before setting the codepage. I assume it'd be safe to just set it every time, but adding the check ensures it doesn't affect anyone other than Windows users.

Thanks!