Electric quote mode: Conditionally replace " (Bug#24710)

Electric quote mode: Conditionally replace " (Bug#24710)

* lisp/electric.el (electric-quote-replace-double): New user option.
(electric-quote-post-self-insert-function): Use it.

* test/lisp/electric-tests.el (electric-quote-replace-double-disabled)
(electric-quote-replace-double-bob)
(electric-quote-replace-double-bol)
(electric-quote-replace-double-after-space)
(electric-quote-replace-double-after-letter)
(electric-quote-replace-double-after-paren): New unit tests.

* doc/emacs/text.texi (Quotation Marks): Document
'electric-quote-replace-double'.