Avoid letting logs with '\r's corrupt the output

\n doesn't really mess up the output, but it made sense to do the same thing for fields that we do
for the message itself.  This does make things like stacktrace keys harder to read, so we might want
to make this configurable.

Fixes #2.

Before:
```
$ (for i in `seq 1 3`; do echo '{"ts":1,"level":"info","msg":"foo\r\n","key":"foo\n","another":"foo\rbar"}'; done) | jlog
↩ key:foo 31 19:00:01 foo
barother:foo
↩ key:↑ another:↑0:01 foo
↩ key:↑ another:↑0:01 foo
  3 lines read; no parse errors.
```

After:
```
$ (for i in `seq 1 3`; do echo '{"ts":1,"level":"info","msg":"foo\r\n","key":"foo\n","another":"foo\rbar"}'; done) | go run ./cmd/jlog
INFO  Dec 31 19:00:01 foo←↩ key:foo↩ another:foo←bar
INFO  Dec 31 19:00:01 foo←↩ key:↑ another:↑
INFO  Dec 31 19:00:01 foo←↩ key:↑ another:↑
  3 lines read; no parse errors.
```

pkg/parse/default_formats.go

@@ -88,8 +88,14 @@ func (f *DefaultOutputFormatter) FormatTime(s *State, t time.Time, w *bytes.Buff
 	s.lastTime = t
 }
 
+func cleanupNewlines(msg string) string {
+	msg = strings.ReplaceAll(msg, "\n", "↩")
+	msg = strings.ReplaceAll(msg, "\r", "←")
+	return msg
+}
+
 func (f *DefaultOutputFormatter) FormatMessage(s *State, msg string, highlight bool, w *bytes.Buffer) {
-	msg = strings.Replace(msg, "\n", "↩", -1)
+	msg = cleanupNewlines(msg)
 	if highlight {
 		msg = f.Aurora.Inverse(msg).String()
 	}
@@ -137,6 +143,7 @@ func (f *DefaultOutputFormatter) FormatField(s *State, k string, v interface{},
 	var value []byte
 	switch x := v.(type) {
 	case string:
+		x = cleanupNewlines(x)
 		value = []byte(x)
 	default:
 		var err error