Support SPARK_MIN/SPARK_MAX env variables.

Hi,

This tiny patch adds support for forcing min/max values based on environment variables.

without min/max (the drawing might look bad in the git markdown, but on the console it's better).

```sh
$ seq 10 20 | spark 
▁▂▂▃▃▅▅▇▇
```

with min/max:

```sh
$ seq 10 20 | SPARK_MIN=0 spark 
▃▃▅▅▇▇██

$ seq 10 20 | SPARK_MAX=30 spark 
▁▁▂▂▂▃▃▃

$ seq 10 20 | SPARK_MIN=0 SPARK_MAX=30 spark 
▅▅▅▅▇▇▇▇
```

The goal is to be able to plot average load over the last hour (or any other period).
and without setting the min/max, it will produce the same chart if it was all 1 or all 0.

```sh
 sar -q |
   awk '$4 ~ /^[0-9]+/ {print $4*100}' |
   tail -n 24 |
   SPARK_MIN=0 SPARK_MAX=$((100*$(nproc))) ./spark
```

thanks for this nice script,
 - gordon