tab completion for *sage* works when the cell is in a non-sage mode

Easiest is to disable tab completion for sage when in a non-sage mode.  Even better is to use Sage's completion mechanism for non-sage modes.  To get the commands for R, for example, do `r.trait_names()`.  For maxima, do `maxima.trait_names()`, etc.  I don't this works for all modes, though.  For example `octave.trait_names()` doesn't work, so we'd have to catch those cases.