Add support for reading logs extra attrs

The jsonlog logger currently allows specifying envs and labels that
should be propagated to the log message, however there has been no way
to read that back.

This adds a new API option to enable inserting these attrs back to the
log reader.

With timestamps, this looks like so:
```
92016-04-08T15:28:09.835913720Z foo=bar,hello=world hello
```

The extra attrs are comma separated before the log message but after
timestamps.

Without timestaps it looks like so:
```
foo=bar,hello=world hello
```

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

jsonlog/jsonlog.go

@@ -15,6 +15,8 @@ type JSONLog struct {
 	Stream string `json:"stream,omitempty"`
 	// Created is the created timestamp of log
 	Created time.Time `json:"time"`
+	// Attrs is the list of extra attributes provided by the user
+	Attrs map[string]string `json:"attrs,omitempty"`
 }
 
 // Format returns the log formatted according to format