(bibtex-entry-format): New options `whitespace', `braces', and `string'.

(bibtex-entry-format): New options `whitespace', `braces', and `string'.
(bibtex-field-braces-alist, bibtex-field-strings-alist)
(bibtex-field-braces-opt, bibtex-field-strings-opt)
(bibtex-cite-matcher-alist): New variables.
(bibtex-font-lock-keywords): Use bibtex-cite-matcher-alist.
(bibtex-flash-head): Use blink-matching-delay.
(bibtex-insert-kill, bibtex-mark-entry): Use push-mark.
(bibtex-format-entry, bibtex-reformat): Handle new options of
bibtex-entry-format.
(bibtex-field-re-init, bibtex-font-lock-cite, bibtex-dist): New
functions.
(bibtex-complete-internal): Do not display messages while
minibuffer is used.  Do not leave around a completions buffer
that is out of date.
(bibtex-copy-summary-as-kill): New optional arg.
(bibtex-font-lock-url): New optional arg no-button.
(bibtex-find-crossref): Use `bibtex-cite-matcher-alist'.
(bibtex-url): Allow multiple URLs per entry.