Missing a doc overhaul and tests, but this deals with most of the
feature upgrades in the blueprint.
The shell version of pt-diskstats is now in bin has pt-diskstats-shell,
in case that's needed.
Mostly bits and pieces at the moments. ReadKeyMini is a portable-ish
Term::ReadKey. Used here by DiskstatsMenu, which is currently only
for show.
Diskstats has most of the logic of the old pt-diskstats, with parts
now implemented by the DiskstatsGroupBy* classes; The latter subclass
the former.
As mentioned at the start, this is a work in progress. In addition to to
not having all the parts attached, also missing are the data-gathering mode,
the command-line argument parsing, and a large amount of tests.
