Add knit_print method for webshot

[jimhester]

This lets you include webshot() calls directly in knitted documents and automatically show the output image.

The object must be returned visibly in order to show up when knitting, so I had to define a print method as well to preserve the previous behavior.

[wch]

I think you can avoid importing knitr. See https://github.com/ramnathv/htmlwidgets/blob/master/R/knitr-methods.R for an example.

[jimhester]

I actually had seen that code when I was doing this originally, thought you might not want the added complexity.

8ae7e27 keeps knitr in Suggests and uses the referenced functions.

[yihui]

I guess this does not need to be exported.

[wch]

Could you also update the functions in image.R to check for the webshot class and also return a filename with that class? Otherwise, I think that doing something like webshot() %>% resize() in a knitr document won't result in the image being put inline.

[jimhester]

@wch Ok both shrink and resize now also return webshot objects as well.

[wch]

Thanks!