Add `interval` options to `iostat-extended-metrics`

Allow the user to extend the `interval` over which statistics are
measured.  This can smooth bursty averages resulting from the default 1
second interval.  Though it's questionable whether this is a good idea,
users should have the option.
Merge pull request #290 from pdf/add_interval_to_iostat

Add `interval` options to `iostat-extended-metrics`