improvements to html_notebook format
improvements to html_notebook format

- Add html_notebook as global tools:rstudio variable so it can be picked up from rmarkdown::render
- Post processor to rename .html to .nb.html
- Use rmarkdown::output_format to create format (rather than direct call to rmarkdown::html_document)
- Expose overridden options in signature of html_notebook (so users can change the defaults)

@kevinushey

jrnold/rstudio,jrnold/rstudio,jar1karp/rstudio,jar1karp/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,jrnold/rstudio,jar1karp/rstudio,jrnold/rstudio,jrnold/rstudio,jrnold/rstudio,jar1karp/rstudio,jar1karp/rstudio,jrnold/rstudio,jar1karp/rstudio,jar1karp/rstudio,jar1karp/rstudio,JanMarvin/rstudio,JanMarvin/rstudio,jrnold/rstudio,jar1karp/rstudio,JanMarvin/rstudio,jrnold/rstudio,JanMarvin/rstudio,JanMarvin/rstudio