Custom rmarkdown format for rendering Rmd files with ggvis content

This is a short-term solution until get the right hooks in knitr to allow embedding ggvis into the standard html_document format
mandate self_contained (this currently won't work with *_files, but will eventually)
Code cleanup
Merge pull request #96 from rstudio/feature/rmarkdown-format

Custom rmarkdown format for rendering Rmd files with ggvis content