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>

docs/reference/commandline/logs.md

@@ -14,6 +14,7 @@ parent = "smn_cli"
 
     Fetch the logs of a container
 
+      --details                 Show extra details provided to logs
       -f, --follow              Follow log output
       --help                    Print usage
       --since=""                Show logs since timestamp
@@ -36,6 +37,10 @@ The `docker logs --timestamps` command will add an [RFC3339Nano timestamp](https
 log entry. To ensure that the timestamps are aligned the
 nano-second part of the timestamp will be padded with zero when necessary.
 
+The `docker logs --details` command will add on extra attributes, such as
+environment variables and labels, provided to `--log-opt` when creating the
+container.
+
 The `--since` option shows only the container logs generated after
 a given date. You can specify the date as an RFC 3339 date, a UNIX
 timestamp, or a Go duration string (e.g. `1m30s`, `3h`). Besides RFC3339 date

man/docker-logs.1.md

@@ -30,6 +30,9 @@ logging drivers.
 **--help**
   Print usage statement
 
+**--details**=*true*|*false*
+   Show extra details provided to logs
+
 **-f**, **--follow**=*true*|*false*
    Follow log output. The default is *false*.
 
@@ -55,6 +58,10 @@ epoch or Unix time), and the optional .nanoseconds field is a fraction of a
 second no more than nine digits long. You can combine the `--since` option with
 either or both of the `--follow` or `--tail` options.
 
+The `docker logs --details` command will add on extra attributes, such as
+environment variables and labels, provided to `--log-opt` when creating the
+container.
+
 # HISTORY
 April 2014, Originally compiled by William Henry (whenry at redhat dot com)
 based on docker.com source material and internal work.