Makefile for rendering R lessons from R Markdown

To fix #524.

This is the start to automating the process of knitting R Markdown files and rendering them to html. It preserves the original YAML header contained in the R Markdown file so that it can subsequently be built with Jekyll.

Currently only the first two novice R lessons (00 and 01) are updated in this PR because a bug in lesson 02 prevents it from knitting. I'll continue updating once this approach is approved. To preview the result, run `make site` and open `_site/novice/r/00-first-timers.html` and/or `_site/novice/r/00-starting-with-data.html`.

Lastly, I also centralized the knitr options by creating one file that all the lessons source. This makes it easier for an instructor to make a global change to all the lessons.