add 2nd attempt impl. with demo

Clearly much simpler than the first attempt -- was completely missing the obvious approach of writing to articles/ which @jjallaire pointed out.

The only way I can think of to circumvent the copy in the Makefile is at the knitr level and I don't know enough about which knitr functions are doing what (and can't glean it quickly enough).
There's one other problem -- if multiple Rmd files include unnamed chunks then they will overwrite each other's figures. I think you need to do something to derive the fig.path from the basename of the input Rmd.