add eventTransformer to logger

[HazemSayad]

fix #116 and should fix #11

Added ability to transform LogEvents using a simple callback

example:

typedef EventTransformerCallback<E extends LogEvent> = E Function(
    LogEvent event);

  Logger({
    LogFilter? filter,
    LogPrinter? printer,
    LogOutput? output,
    Level? level,
    EventTransformerCallback? eventTransformer,
  })

Open for feedback.

If this feature is not a good fit with the vision of the project please let me know if i can modify the code to make it fit, or if it is out right not a feature this project should have

Let me know if the supporting docs and tests i added are up to standard or you would like me to modify it