Allow switching out LogFilter globally

Fixes #33

Author: yangsfang

lib/src/logger.dart

@@ -50,6 +50,9 @@ class Logger {
   /// All logs with levels below this level will be omitted.
   static Level level = Level.verbose;
 
+  /// The current default implementation of log filter.
+  static LogFilter defaultLogFilter = DevelopmentFilter();
+
   static final Set<LogCallback> _logCallbacks = {};
 
   static final Set<OutputCallback> _outputCallbacks = {};
@@ -69,7 +72,7 @@ class Logger {
     LogPrinter? printer,
     LogOutput? output,
     Level? level,
-  })  : _filter = filter ?? DevelopmentFilter(),
+  })  : _filter = filter ?? defaultLogFilter,
         _printer = printer ?? PrettyPrinter(),
         _output = output ?? ConsoleOutput() {
     _filter.init();