Add support for performance data

[rchekaluk]

Using Nagios graphs requires that plugins emit performance data (see also here). Add basic support for performance data to this framework. (a companion contribution will be coming shortly for check_docker).

Apologize if any design or coding missteps; this is my first work with golang.

[agimenez]

Hey, can you check this PR out? 3 years look like a long time :)

[CLAassistant]

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