Implement knit_print S3 method

This changes the way ggvis objects are printed from within knitr. By implementing knit_print, ggvis objects now automatically render correctly even without results = 'asis'.

Their dependencies are also automatically included so there is no longer a need for special ggvis rmarkdown formats (standard formats like html_document, ioslides_presentation, etc. can be used).

Note that this requires the most recent development versions of knitr and rmarkdown to work correctly.