Added command for knitting Rmd/rmd files using knitr. It will also work with .md (or other redcarpet.markdown files, but it will then create a .md.txt file or equivalent instead of the .md file). Using knitrBootstrap is recommended as described at http://lcolladotor.github.io/2013/12/10/knitrBootstrap/ but the command is included to show how to use knitr and/or for those who prefer this option.