Added template for PLoS journals (LaTeX).

I made a template for PLoS journals (LaTeX). As far as I can tell they all use the same formatting for submissions. It's my first template for Rmarkdown and I built it based on the other templates present in this package. I hope I've done everything correctly :-) 
You could add header-includes (so users can easily include their own macros). That would look like this: https://github.com/rstudio/rmarkdown/blob/master/inst/rmarkdown/templates/tufte_handout/resources/tufte-handout.tex#L52-L54 and then require an "includes" argument be added to the plos_article function (and forwarded to pdf_document)