add an example of R HTML vignettes
slight tweaks to the Rmd vignette
only use the first element of 'input' in case of multiple input files
now we can also embed pandoc configurations in a markdown file
tweak documentation of pandoc()
only support unique output format at the moment; may remove this restriction in future

e.g. latex can mean .tex or .pdf
when config is empty, assign character(0) to it instead of halting
when there is only one field in config, should drop(cfg) to make it a vector
return early when x is of length 0 or all elements are NA
roxygenize
bump version; I think this is pretty much what I have for R 3.0