Add External Command metrics

[wbollock]

External commands in the last 1/5/15 mins would be a nice metric.

External Commands Last 1/5/15 min:      35541 / 86557 / 88951

Sadly I don't see it exposed in the API (would expect it in system/statusdetail). So this will be a nagiostats only metric. I need to rework things to where you can have both nagiostats and the NagiosXI API key collection methods.