Allow for width/height setting when specifying an external image via markdown

[coatless]

When trying to add an image sitting in man/figures, I'm running into a problem with specifying height/width constraints.

This works:

![](pkg-overview.png "Package Overview")

But, adding width or height keys causes the markdown to leak through, e.g. we in the Rd:

![](pkg-overview.png "Package Overview" width="500px")

From 2.7 Figures, I think at least width can be specified using regular \figure markup, e.g.

\if{html}{\figure{Rlogo.svg}{options: width=100 alt="R logo"}}
\if{latex}{\figure{Rlogo.pdf}{options: width=0.5in}}

Would it be possible to expose a width key on images? I think this would involve changing:

roxygen2/R/markdown.R

Lines 422 to 424 in 2357de6

	mdxml_image <- function(xml) {
	dest <- xml_attr(xml, "destination")
	title <- xml_attr(xml, "title")

And updating documentation here:

roxygen2/vignettes/rd-formatting.Rmd

Lines 194 to 202 in 2357de6

	### Images
	Markdown syntax for inline images works.
	The image files must be in the `man/figures` directory:
	``` r
	#' Here is an example plot:
	#' 
	```