Major Logging Overhaul

in this commit:

- centralize logging output handler.
- set global Info/Debug/Error log levels based on config file or flags.
- remove per-plugin debug arg handling.
- add a I!, D!, or E! to every log message.
- add configuration option to specify where to send logs.

closes #1786

logger/logger.go

@@ -0,0 +1,58 @@
+package logger
+
+import (
+	"io"
+	"log"
+	"os"
+
+	"github.com/influxdata/wlog"
+)
+
+// newTelegrafWriter returns a logging-wrapped writer.
+func newTelegrafWriter(w io.Writer) io.Writer {
+	return &telegrafLog{
+		writer: wlog.NewWriter(w),
+	}
+}
+
+type telegrafLog struct {
+	writer io.Writer
+}
+
+func (t *telegrafLog) Write(p []byte) (n int, err error) {
+	return t.writer.Write(p)
+}
+
+// SetupLogging configures the logging output.
+//   debug   will set the log level to DEBUG
+//   quiet   will set the log level to ERROR
+//   logfile will direct the logging output to a file. Empty string is
+//           interpreted as stdout. If there is an error opening the file the
+//           logger will fallback to stdout.
+func SetupLogging(debug, quiet bool, logfile string) {
+	if debug {
+		wlog.SetLevel(wlog.DEBUG)
+	}
+	if quiet {
+		wlog.SetLevel(wlog.ERROR)
+	}
+
+	var oFile *os.File
+	if logfile != "" {
+		if _, err := os.Stat(logfile); os.IsNotExist(err) {
+			if oFile, err = os.Create(logfile); err != nil {
+				log.Printf("E! Unable to create %s (%s), using stdout", logfile, err)
+				oFile = os.Stdout
+			}
+		} else {
+			if oFile, err = os.OpenFile(logfile, os.O_APPEND|os.O_WRONLY, os.ModeAppend); err != nil {
+				log.Printf("E! Unable to append to %s (%s), using stdout", logfile, err)
+				oFile = os.Stdout
+			}
+		}
+	} else {
+		oFile = os.Stdout
+	}
+
+	log.SetOutput(newTelegrafWriter(oFile))
+}