Check pandoc version on startup, give informative message

[noamross]

Per #44, it would be helpful to check the version of Pandoc on package load, at least in interacctive mode. Run rmarkdown::pandoc_version(), and give an informative message if version is <2.1.2. One could also use rmarkdown::pandoc_exec() to figure out if the Pandoc version is the one bundled with RStudio, in which case the message can recommend updating RStudio.

[noamross]

Check that RSTUDIO_PANDOC exists and if it is the same and pandoc_exec() to see if the bundled version is being used.