Added integration with R markdown

The proposed changes allow R source highlighting in fenced code chunks and inline R code*. This allows users to then evaluate code using the `R` bundle. There are two new proposed commands that allow 'knitting' a R Markdown file to create the HTML output. 'knitting' involves evaluating in R the R code chunks and creating a markdown file (if the original file is .Rmd or .rmd, otherwise .md -> .md.txt to avoid overwriting) with the results from the R code evaluation. One of the command uses the main package for doing so "knitr" while the other one (which I personally recommend) uses "knitrBootstrap" as I explain in this [blog post](http://lcolladotor.github.io/2013/12/10/knitrBootstrap/). 

* The inline R code is not yet completely functional and I need help finishing this. Described at https://github.com/streeter/markdown-redcarpet.tmbundle/issues/4