don't let R completion manager preview keypress when not in R mode

This forces the RCompletionManager to verify that the cursor is in R mode before attempting to produce automatic R completion popups.

Note that this just disables auto-popup in the 'tex' mode of `.Rnw` documents; the user can still explicitly request R completions outside of an R chunk. I haven't yet identified where the R cursor check is made for `.Rmd` files, since you cannot request R completions outside of an R chunk there.