Update cslpreview.xul

@dstillman, this pull request replaces #7. The changes work in my setup. The main functional change is that the citation-format filter now works for CSL 1.0 styles, since the code hadn't yet been updated to find the "citation-format" attribute on cs:category.

I separated the changes as much as possible for easy review. Let me know if I should squash the commits.