GeoJSON code tweaked and added to Rmd source

Made some small changes to your functions (e.g., take an ID rather than a link, and moved link parsing out of functions). Added a rebuild option. Moved the code to the Rmd source so it's compiled and displayed in the master/README.md. Links are created to the geoJSON files but I still can't figure out how to embed the geoJSON file into the README.md and have it display in browser. Contacted github about this.