Emit performance data

Emit [performance data](http://docs.pnp4nagios.org/pnp-0.6/about#system_requirements) (see also [here](https://nagios-plugins.org/doc/guidelines.html#AEN200)) for consumption by Nagios pnp4graph. Changes the following normal output:
```
$ ./check_docker -base-url="http://localhost:1234"
OK: 1 containers
```

to this:
```
$ ./check_docker -base-url="http://localhost:1234"
OK: 1 containers | 'containers'=1;;;;
```

Requires go_nagios support https://github.com/newrelic/go_nagios/pull/1