Add a -z flag to suppress datasets with no activity

[Freaky]

Fixes #12

This feels best combined with -s name, as the multi-line naming gets a bit too dynamic.

[Freaky]

Thoughts on making -s name default in this mode?

[jimsalterjrs]

Hmmm, there appears to have been a bug in this PR, if you tested after I merged all the others today:

me@banshee:/usr/local/bin$ sudo ioztat
  File "/usr/local/bin/ioztat", line 68
    header, _fieldnames, *fields = [line.split() for line in f]
                         ^
SyntaxError: invalid syntax

[jimsalterjrs]

well, it doesn't appear to have been this one--I'm seeing the breakage after reverting this as well.

[jimsalterjrs]

@Freaky I don't think I have much of an opinion about using -s name by default when -z is on. That seems fine to me. I suppose the question is whether people who want to opt out can specify a -s argument that brings back the default sorting?

Assuming the answer to the latter is "yes" I'd say you can go ahead and change the default sort when -z is on.