It seems a bit weird that this extends FileAppender. Why not instead have a `RollingFileAppender` just encapsulate multiple instances of a `FileAppender`. Logically, a rolling appender is not really a sub type of an appender that appends to one file.

The fact that `outputFile` is a `var` makes the semantics of the `FileAppender` pretty confusing. Because a single FileAppender will be used to append to many files. It seems more intuitive to have the rolling appender simply create a new FileAppender when a new file is created. You'd just need to make sure you stop and flush the old appender before switching over.