Rewrite iostat-extended-metrics plugin

Both the currently existing iostat plugins were borken in some way
(iostat-extended-metrics produced no output, and iostat-metrics
erroneously outputs headers as metrics).  Additionally neither of them
includes the `avg-cpu` stats, which can be useful for
alerting/diagnosis.

This updated plugin has more robust parsing (should not break with
banners, minor formatting changes, or added/removed fields), and
includes `avg-cpu` stats.  All included fields are dynamically generated
from output headers.