[PATCH] Add krona-tools.

[Ben Woodcroft]

This is a neat little visualisation tool that generates zoomable HTML5 charts
from hierarchical data e.g. disk usage
http://marbl.github.io/Krona/examples/du.krona.html

It is mainly used for bioinformatics (specifically metagenomics) I believe,
but is more generally applicable.  Not sure if web.scm is the right place for
it?

Thanks in advance,
ben