Add iostat-full-metrics plugin

Both the currently existing iostat plugins were borken in some way
(iostat-extended-metrics produces 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 plugin has more robust parsing (should not break with banners,
minor formatting changes, or added/removed fields), and includes
`avg-cpu` stats.  Included fields are dynamically generated from output.

I wasn't sure whether to overwrite one of the existing plugins, but
since this is a total rewrite, I thought it better to err on the side 
of caution.  Happy to do some renaming if preferred.