Aggressive Prerender checks within rmarkdown

[schloerke]

Fixes #169

Uses rstudio/rmarkdown#1420 to rerender tutorials if any html dependency package version is different and if any package used to build the document is different at run time.

Because this is so aggressive and the checks should be done every time a user runs the document, setting the environment variable RMARKDOWN_RUN_PRERENDER=0 should not be done.

[schloerke]

Will require a followup PR to remove the reference in the DESCRIPTION file once rmarkdown is released to CRAN

[jjallaire]

One scenario we should be sure to encompass is to be able to run tutorials in an environment where they are never rendered (we need this e.g. for Shiny Server when they are deployed in a readonly directory). Is that still possible?

[schloerke]

rmarkdown still has the hooks for skipping the pre-render.

Can still execute this to never prerender:

withr::with_envvar(c(RMARKDOWN_RUN_PRERENDER = "0"), {	    
  learnr::run_tutorial(FILE, package = PKG)
})

Would it be better to have a run_tutorial_local function that implements the always possibly render functionality? (Keeping run_tutorial as is)

learnr::run_tutorial_local(FILE, package = PKG)

[jjallaire]

No, I think it's fine as long as rmarkdown has that hook.

[wch]

0.9.2.9000