Support for entering ellipsis in electric terminator mode

[Robin Green]

When electric-terminator is on, it's unclear how to enter an ellipsis (...) to invoke the default tactic in a "Proof with [tactic]" in Coq, except by disabling electric-terminator and re-enabling afterwards, which is inconvenient.

[David Aspinall]

I've added a patch which allows you to use a numeric prefix just as with the ordinary self-insert. If a prefix is given, the electric action will not happen. So "M-3 ." will insert the ellipsis, "M-1 ." forces insert of ".".

Of course, you could just type "C-q ." three times, defined your own command, etc. I'm not sure which of these is least unobvious!