Makefile for converting R Markdown files to Markdown

See #524 for discussion. See #598 for my previous attempt.

To observe the effect of these changes, run the following:

    make rmarkdown
    make clean
    make site

And then preview any of the lessons, e.g. `_site/novice/r/01-starting-with-data.html`. The style should be consistent with the other novice lessons, e.g. `_site/novice/python/01-numpy.html`.