Sample implementation for issue 78

An approach to tackle issue #78, I added a bufferedSink to handle treshhold loggers.

Everything will be logged to the Console, but when there is an Error level, the last 3 logevents will be published to the ColoredConsole too.

```csharp
 Log.Logger = new LoggerConfiguration()
                .MinimumLevel.Debug()
                .WriteTo.ThresholdLogger(3, LogEventLevel.Error, lc=>lc.WriteTo.ColoredConsole(LogEventLevel.Information))
                .WriteTo.Console(
                    outputTemplate: "{Timestamp:HH:mm:ss} ({ThreadId}) [{Level}] {Message:l}{NewLine:l}{Exception:l}")
                .CreateLogger();
```

It requires some more work, but it might be an idea for what is described in #78.