Auto-setup for bug-reference-mode

Auto-setup for bug-reference-mode

Tries to guess suitable bug-reference-bug-regexp and
bug-reference-url-format values based on version control URL (in file
buffers) and mail information (in Gnus summary and article buffers).

* lisp/progmodes/bug-reference.el
(bug-reference--maybe-setup-from-vc): New defun.
(bug-reference-setup-from-vc-alist): New defvar defining setup rules
based on version control URL.
(bug-reference-try-setup-from-vc): New defun using above defvar.
(bug-reference--maybe-setup-from-mail): New defun.
(bug-reference-setup-from-mail-alist): New defvar defining setup rules
based on mail/newsgroups and header values.
(bug-reference-try-setup-from-gnus): New defun using above defvar.
(bug-reference--try-setup-gnus-article): New defun.
(bug-reference--run-auto-setup): New defun.
(bug-reference-mode): Call bug-reference--run-auto-setup as
:after-hook.
(bug-reference-prog-mode): Call bug-reference--run-auto-setup as
:after-hook.