Vignette sources

Because redux.Rmd requires Redis to be run, we use a trick (due to Carl Boettiger; see https://github.com/ropensci/redux/pulls/6) to avoid building the vignette on CRAN. Running make vignettes (at the top level) will:

• Knit vignettes_src/redux.Rmd -> vignettes_src/redux.md
• Copy vignettes_src/redux.md to vignettes/redux.Rmd
• Knits vignettes/redux.Rmd (which has no executable code) to inst/doc/

The R CMD check vignette builder then repeats the last step which does not require Redis server to be present.