emacs/notmuch-address.el: add notmuch-address-selection-function

emacs/notmuch-address.el: add notmuch-address-selection-function

Added a customizable variable notmuch-address-selection-function
and the function with the same name to provide a way for user to
change the function called to do address selection.

By default the functionality is exactly the same as it has been so
far; completing-read is called with the same parameters as before.

Setting equivalent lambda expression in place of using
notmuch-address-selection-function function is done as follows:

(setq notmuch-address-selection-function
   (lambda (prompt collection initial-input)
     (completing-read prompt collection nil nil initial-input)))

For example drop-in replacement with ido-completing-read can be done
easily as an one alternative to the default.