Add library and command for human-readable logs (#9809)

* Update logrus to 1.2 and add as a direct dependency

* Create an mlog/human package for pretty-printing logs

This package can read JSON logs from mattermost.log, and output the data to
either logrus or a custom formatter, to make the logs more human readable.

* Create a command for outputting human-readable logs

This command will read JSON data from mattermost.log or stdin, and
output in a human readable format. An optional argument can be used
to activate logrus output (which includes color support).

* Reorganize code in mlog/human and improve logrus timestamp formatting

mlog/human/process.go

@@ -0,0 +1,23 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package human
+
+import (
+	"bufio"
+	"io"
+)
+
+type LogWriter interface {
+	Write(e LogEntry)
+}
+
+// Read JSON logs from input and write formatted logs to the output
+func ProcessLogs(reader io.Reader, writer LogWriter) {
+	scanner := bufio.NewScanner(reader)
+	for scanner.Scan() {
+		s := scanner.Text()
+		e := ParseLogMessage(s)
+		writer.Write(e)
+	}
+}