[command]: Implement a command to render documents using rmarkdown/knitr

[fmoralesc]

We can use both knitr and rmarkdown. Pipeline should be configurable (g:rmarkdown#pipeline).

Basically, for rmarkdown we should execute

R -e 'rmarkdown::render('input.Rmd')

This needs a way to pass extra arguments to rmarkdown::render(), like in

render("input.Rmd", pdf_document(latex_engine = "lualatex"))

For knitr, we should run

R -e 'library(knitr); knit(...)'

this also needs a way to pass arguments to knit(...).

[fmoralesc]

Done.