Add support for performance data

Using Nagios graphs [requires that plugins emit performance data](http://docs.pnp4nagios.org/pnp-0.6/about#system_requirements) (see also [here](https://nagios-plugins.org/doc/guidelines.html#AEN200)). Add basic support for performance data to this framework. (a companion contribution will be coming shortly for [check_docker](https://github.com/newrelic/check_docker)).

Apologize if any design or coding missteps; this is my first work with golang.