Add ability to render rmd

[ljwoodley]

First draft of issue #83

[ljwoodley]

@ChemiKyle @pbchase any objections to the file name and locations chosen? If not I'll create a new section for the documentation at https://github.com/ctsit/redcapcustodian/blob/master/docs/custom_rscript.md.

[pbchase]

This looks good. You took this further than I imagined and I think it was the right thing to do. Please go ahead and add the new section to the custom_rscript.md