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).
Couldn't this just be '../articles/figure' then you could get rid of the changes to Makefile and _config.yml ?