Support backtab to move backwards through the completion list

Tab moves forward in the list; Shift-Tab for backwards would be very convenient and doesn't look like it would take too much extra code.

As far as I can tell, backtab (Shift-Tab) sends `[Z` (ANSI `CBT`) on VT100-compatible terminals that distinguish Tab from Shift-Tab. This is true on mintty (in my testing), is [listed on vt100.net](https://vt100.net/docs/vt510-rm/CBT.html), is [listed on invisible-island](http://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h2-Functions-using-CSI-_-ordered-by-the-final-character_s_), and is at least [acknowledged in Windows consoles](https://docs.microsoft.com/en-us/windows/console/console-virtual-terminal-sequences#span-idtabsspanspan-idtabsspanspan-idtabsspantabs).