allow more stuff to affect cache

[yihui]

what are described in the main manual and http://yihui.name/knitr/demo/cache/ may still not be enough; reproducible research can require even more, e.g. the version of R and even versions of all R packages, the values of environmental variables, all options() and so on; any changes to them may need to invalidate the cache, and we need to make it possible to allow users customize this list of objects

why does this make sense? see http://simplystatistics.tumblr.com/post/17550711561/duke-clinical-trials-saga-on-60-minutes-first

[github-actions]

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