odt-utils, needed by dtbook-to-odt

I'd like to share the dtbook-to-odt script I've been working on, if you think it's good enough. Of course the SBS specific stuff has to be factored out, which means I'll end up with two version of the scripts: dtbook-to-odt and sbs:dtbook-to-odt. The main XSLT file was already split out into a general purpose one and an SBS specific one that extends it. And now I've also moved some more functionality to this odt-utils, in order to have less code duplication.

The latest version of dtbook-to-odt can be found at: https://github.com/sbsdev/sbs-pipeline/tree/master/dtbook-to-odt