added template helper stack_outline.php

[georgehansper]

The file stack_outline.php is a template helper.

It makes it very easy to combine multiple Data Sources into a single graph, either stacked or not.
Templates can be reduced to 2 lines of php, like this:

$opt[1] = "--vertical-label "bytes" --title "Disk usage for $hostname" -l 0 ";
require('stack_outline.php');

The above example is included in the comments.

I can provide further, more sophisticated usage examples.