Add support for rendering ggvis output

[Keith Hughitt]

ggvis plots can now be rendered to the vanilla html_document rmarkdown class, e.g.

https://github.com/rstudio/ggvis/blob/master/vignettes/interactivity.Rmd

Currently, the rendering fails, however, in knitrBootstrap.

The issue appears to be related to some inline scripts that are required not being inserted properly; comparing the output from html_document and knitrBootstrap::bootstrap_document.

[Jim Hester]

Did you try this with #68, it should work with that, at least it did when I made http://jimhester.github.io/ggplot2ToGgvis/. If it is failing with that I will have look into it. Now that rmarkdown is using Bootstrap 3 I can simplify some things on the knitrBootstrap side, which should iron out more of these issues.