[FEATURE REQUEST] Add the capability to detect zombie processes with sensu-processes-check

[jhenderson-pro]

Feature Suggestion

As a Sensu operator, I'd like to be able to detect zombie processes using the sensu-processes-check.

Possible Implementation

Add the parts of the gopsutil library that can detect the zombie states.

Context

A real-world example (and specific to this case) will be to evaluate if the OpenVPN process has entered a zombie state.

[jhenderson-pro]

This has already been requested: sensu/sensu-processes-check#9