Support for knitr and RMarkdown

[vh-d]

Support for RMarkdown documents may be implemented in several ways.

One way is to create a rmarkdown class (subclass of file_node probably). This would take a RMarkdown document and inject code loading its dependencies at the beginning of the script.

Another way is to use current file_node and implement API to access values of dependencies from RMarkdown documents easily (without loading the whole Rflow etc...).