Add customization options for dictionary-search

Add customization options for dictionary-search

Allow users to customize 'dictionary-search' via several new
customization options.

* lisp/net/dictionary.el (dictionary-define-word)
(dictionary-match-word, dictionary-completing-read-word)
(dictionary-dictionaries, dictionary-completing-read-dictionary)
(dictionary-display-definition-in-help-buffer): New functions.
(dictionary-read-word-prompt)
(dictionary-display-definition-function)
(dictionary-read-word-function)
(dictionary-read-dictionary-function)
(dictionary-search-interface): New user options.
(dictionary-search): Use them.
(dictionary-read-dictionary-default)
(dictionary-read-word-default): New functions, extracted from
'dictionary-search'.
* etc/NEWS: Announce.