Rudimentary DOI support

Here's a gist of a modified Harvard Anglia XSL file that demonstrates how DOI support could be implemented into the guts of BibWord to allow all styles to display DOI numbers (tbf they're only really relevant for journal articles but it would be nice). I would make a PR but I can't figure out where I should put the changes.

https://gist.github.com/CiaraOBrien/60e5d5f444c458e790a1dec34f541801