Tweaks requested by Baron, plus fixes.

Tweaks:
* --headers command line option, which can be used to
        enable/disable the automatic headers, and the
        automatic empty lines between samples.
* --show-timestamps, which changes the #ts line with
        a timestamp in the HH:MM:SS format.
* Always use ReadKeyMini, even when we have Term::ReadKey.
        In the latter case, use the Term::ReadKey functions,
        but if they fail, try again with our own versions.

Fix:
Looks like I forgot to actually enable the "save the current, first
and last" samples. This is now on. It should mean faster and better
output when changing --group-by modes.

lib/DiskstatsGroupByDisk.pm

@@ -118,7 +118,12 @@ sub clear_state {
 
 sub compute_line_ts {
    my ($self, %args) = @_;
-   return "{" . ($self->{_iterations} - 1) . "}";
+   if ( $self->show_timestamps() ) {
+      return $self->SUPER::compute_line_ts(%args);
+   }
+   else {
+      return "{" . ($self->{_iterations} - 1) . "}";
+   }
 }
 
 sub delta_against {