OHM Idle Plugin
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The OHM Idle plugin is used to discover when there have been varying lengths
of user input inactivity.

The idle plugin exposes 2 keys:
 * idle.state
 * idle.timeout

When there is user activity, idle.state will be set to 0.
After idle.timeout milliseconds idle.state will be incremented.

If a policy wishes to measure, say 2 different lengths of user inactibity, they
may set timeout to a new value when idle.state is incremented. When that new timout expires, idle.state will be incremented again, and so on.

See defualt policy module 'timeouts' for an example.
