Sorting bibliography

[DunLug]

Hello,
I wrote an IEEE article using the rticles templates and I needed to custom the sorting order of the citations to be 1, 2, 3...
I modified the template this way and added a citation_sorting option in Index.Rmd yaml section. I think there may be a better solution (for example as a subitem of the citation_package option) but I wanted not to modify the previous options as possible.

[yihui]

Could you also add an example of this option in a comment in the YAML frontmatter of inst/rmarkdown/templates/ieee_article/skeleton/skeleton.Rmd?

Please do the same thing for #263 and #264 (add examples in comments, so future users are aware of these possibilities). Thank you very much!

[yihui]

Thanks!