Allow cslpreview.xul to sort by "citation-format"

"Update cslpreview.xul (and style.js) to allow styles to be filtered by "citation-format" (in CSL 1.0 this attribute replaced "term" on cs:category). Also some cosmetic changes."

I had some issues including support for CSL 0.8.1 styles, but I don't know if lack of CSL 0.8.1 support is a problem. I also assumed that the change I made in style.js doesn't cause problems elsewhere, which might not be the case.