in_exec: Add a simple example to fetch load average stats

[fujimotos]

This patch expands the section "Use Cases" to include a much simpler example that shows how to utilize the in_exec plugin.

Two notable points:

1) One can use pipe notations '|' in the command option. 2) <parse> option can used to handle unstructured data (e.g. space-separated values on /proc/loadavg)