#833 The "trace" log level is allowed when debug is specified.

Fixes #833, though in a different way than recommended by @khellang.

@Romoku's problem was that they were not able to use the `trace` log level when using `-debug` which this fixes. However lower log levels (`info` and `error`) will still be converted to `debug` when using the `-debug` flag.

The originally suggested fix of making `LogLevel` nullable had reasonably far reaching consequences, but I think that this is a simpler way of addressing the original issue.