Electric quotes: Improve support for Markdown mode (Bug#24709)

Electric quotes: Improve support for Markdown mode (Bug#24709)

Introduce a new user option 'electric-quote-context-sensitive'.  If
non-nil, have ' insert an opening quote if sensible.

Also introduce a new variable 'electric-quote-code-faces'.  Major
modes such as 'markdown-mode' can add faces to this list to treat text
as inline code and disable electric quoting.

* lisp/electric.el (electric-quote-context-sensitive): New user
option.
(electric-quote-code-faces): New variable.
(electric-quote-post-self-insert-function): Treat ' as ` if
desired and applicable; disable electric quoting for given faces.

* test/lisp/electric-tests.el (electric-quote-opening-single)
(electric-quote-closing-single, electric-quote-opening-double)
(electric-quote-closing-double)
(electric-quote-context-sensitive-backtick)
(electric-quote-context-sensitive-bob-single)
(electric-quote-context-sensitive-bob-double)
(electric-quote-context-sensitive-bol-single)
(electric-quote-context-sensitive-bol-double)
(electric-quote-context-sensitive-after-space-single)
(electric-quote-context-sensitive-after-space-double)
(electric-quote-context-sensitive-after-letter-single)
(electric-quote-context-sensitive-after-letter-double)
(electric-quote-context-sensitive-after-paren-single)
(electric-quote-context-sensitive-after-paren-double)
(electric-quote-markdown-in-text)
(electric-quote-markdown-in-code): New unit tests.