Handling R scripts without conversion into Rmd

[thebioengineer]

Currently {DataPackageR} converts R scripts into Rmd via knitr::purl() to then get run via rmarkdown::render() so they are in a "clean" R environment.

Rather than doing that, we should set up the ability to source the R script in a clean R session and maintain the pulling of objects out of the rendered environment for saving/sharing.