Added disk-stats

I've added disk-stats to show some disk usage info using 'df' output.
Formats allowed:
%p used space (%)
%u used space (in kb)
%f free space (in kb)
%t total space (in kb)

The target partition can be configured with disk-usage-partition.

Not sure if the copyright statement is ok. It's based on memory-stats so it's almost the same.