use min and max values from rrdtool dump

Low and high fence printout values should never exceed legal range, when
such is known.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
use wider indentation

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
add a command line option to specify --min lof and --max hif values

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
be mathematically correct

If input list is long enough this is practically unnecessary, as with the
case of typical rrd dump.  But when playing with a short list from other
input source the correctness does matter.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
improve examples

Signed-off-by: Sami Kerola <kerolasa@iki.fi>