Add optional syslog logrus hook (#343)

* add optional syslog logrus hook

* document syslog

vendor/gopkg.in/mcuadros/go-syslog.v2/format/rfc6587.go

@@ -0,0 +1,45 @@
+package format
+
+import (
+	"bufio"
+	"bytes"
+	"strconv"
+
+	"gopkg.in/mcuadros/go-syslog.v2/internal/syslogparser/rfc5424"
+)
+
+type RFC6587 struct{}
+
+func (f *RFC6587) GetParser(line []byte) LogParser {
+	return &parserWrapper{rfc5424.NewParser(line)}
+}
+
+func (f *RFC6587) GetSplitFunc() bufio.SplitFunc {
+	return rfc6587ScannerSplit
+}
+
+func rfc6587ScannerSplit(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	if atEOF && len(data) == 0 {
+		return 0, nil, nil
+	}
+
+	if i := bytes.IndexByte(data, ' '); i > 0 {
+		pLength := data[0:i]
+		length, err := strconv.Atoi(string(pLength))
+		if err != nil {
+			if string(data[0:1]) == "<" {
+				// Assume this frame uses non-transparent-framing
+				return len(data), data, nil
+			}
+			return 0, nil, err
+		}
+		end := length + i + 1
+		if len(data) >= end {
+			// Return the frame with the length removed
+			return end, data[i+1 : end], nil
+		}
+	}
+
+	// Request more data
+	return 0, nil, nil
+}