Update knit_print.webshot so that images are included at normal size

[wch]

The intro vignette has some code that sizes the screenshots for inclusion in an Rmd document, but it would be nice if that were not necessary.

This will require some changes to knit_print.webshot as well as in the knitr package.

cc @yihui

[vnijs]

Not sure if this is related but ... automatic sizing seems to work fine on mac but not so much on Windows. Using webshot 0.4.1 and the Rmd below the DT table covers the full width of the page on Mac but is very small on Windows (see screenshot of the windows pdf below). Same effect occurs with other htmlwidgets (e.g., DiagrammeR and plotly)

cc @yihui

---
title: "Widget Size"
output:
  pdf_document: default
---

```{r}
DT::datatable(iris)
```

[vnijs]

@wch Can you suggest a workaround for widget screenshot sizing for Rmd-to-Word and/or Rmd-to-PDF on Windows? Although this works fine on Mac, the default size on Windows is so small that it isn't really usable.

cc @yihui

FYI Screenshot on Mac shown below for reference