Minimally invasive change to put logging behind a user-replaceable interface.

This could probably be done better, and there are probably awful caveats
and hidden gotchas with this approach. I REGRET NOTHING.

logging/logging.go

@@ -0,0 +1,43 @@
+package logging
+
+// The IRC client will log things using these methods
+type Logger interface {
+	// Debug logging of raw socket comms to/from server.
+	Debug(format string, args ...interface{})
+	// Informational logging about client behaviour.
+	Info(format string, args ...interface{})
+	// Warnings of inconsistent or unexpected data, mostly
+	// related to state tracking of IRC nicks/chans.
+	Warn(format string, args ...interface{})
+	// Errors, mostly to do with network communication.
+	Error(format string, args ...interface{})
+}
+
+// By default we do no logging. Logging is enabled or disabled
+// at the package level, since I'm lazy and re-re-reorganising
+// my code to pass a per-client-struct Logger around to all the
+// state objects is a pain in the arse.
+var logger Logger = nullLogger{}
+
+// SetLogger sets the internal goirc Logger to l. If l is nil,
+// a dummy logger that does nothing is installed instead.
+func SetLogger(l Logger) {
+	if l == nil {
+		logger = nullLogger{}
+	} else {
+		logger = l
+	}
+}
+
+// A nullLogger does nothing while fulfilling Logger.
+type nullLogger struct{}
+func (nl nullLogger) Debug(f string, a ...interface{}) {}
+func (nl nullLogger) Info(f string, a ...interface{}) {}
+func (nl nullLogger) Warn(f string, a ...interface{}) {}
+func (nl nullLogger) Error(f string, a ...interface{}) {}
+
+// Shim functions so that the package can be used directly
+func Debug(f string, a ...interface{}) { logger.Debug(f, a...) }
+func Info(f string, a ...interface{}) { logger.Info(f, a...) }
+func Warn(f string, a ...interface{}) { logger.Warn(f, a...) }
+func Error(f string, a ...interface{}) { logger.Error(f, a...) }