Allow users to specify stdout and stderr in Logging plugin

lib/src/plugin/logging.dart

@@ -1,5 +1,5 @@
 import 'dart:async';
-import 'dart:io';
+import 'dart:io' as io;
 
 import 'package:logging/logging.dart';
 import 'package:nyxx/src/api_options.dart';
@@ -30,14 +30,27 @@ class Logging extends NyxxPlugin {
   /// Whether to censor the token of clients this plugin is attached to.
   final bool censorToken;
 
+  /// The sink normal messages are sent to.
+  ///
+  /// Defaults to [io.stdout].
+  final StringSink stdout;
+
+  /// The sink error messages are sent to.
+  ///
+  /// Defaults to [io.stderr].
+  final StringSink stderr;
+
   /// Create a new instance of the [Logging] plugin.
   Logging({
     this.stderrLevel = Level.WARNING,
     this.stackTraceLevel = Level.SEVERE,
     this.logLevel = Level.INFO,
     this.truncateLogsAt = 1000,
     this.censorToken = true,
-  });
+    StringSink? stdout,
+    StringSink? stderr,
+  })  : stdout = stdout ?? io.stdout,
+        stderr = stderr ?? io.stderr;
 
   static int _clients = 0;