Bug-reference auto-configuration

[Tassilo Horn]

Hi all,

I've just pushed a feature for bug-reference.el.  When
`bug-reference-mode' or `bug-reference-prog-mode' is activated and after
running its hook still no `bug-reference-url-format' is set, it'll try
to guess suitable values.

Currently, there are 2 such guessing function in place:

- `bug-reference-try-setup-from-vc' tries to guess the right values from
  the URL of the file's version control repository.

  - Savannah git projects get our GNU debbugs URL.

  - GitHub and Gitlab get their issue trackers set with standard #18 bug
    references (and !17 merge requests for Gitlab) and also
    cross-project references such as namespace/project#19 are supported.

  - It is easy to add new rules by adding entries to
    `bug-reference-setup-from-vc-alist'.

- `bug-reference-try-setup-from-gnus' tries to guess the right values in
  Gnus summary and article buffers by matching against the group name
  and mail header values From, To, Cc, and List-ID.

  - Currently, there's just one rule that should set the GNU debbugs URL
    in any Emacs, Gnus, AUCTeX, or Tramp related group or article.

  - Again, it's easy to add new rules by adding entries to
    `bug-reference-setup-from-mail-alist'.

    As the name suggests, that variable is not Gnus-related but I hope
    someone volunteers to add similar setup functions for rmail and
    other emacs mail clients which can use that variable, too.
    Basically, the only MUA-specific thing that needs to be done is to
    somehow get the mail-folder/newsgroup name and/or the interesting
    mail header values From, To, Cc, and List-ID and call
    `bug-reference--maybe-setup-from-mail'.

I'd welcome some more testing.  You just have to enable
`bug-reference-mode' or `bug-reference-prog-mode' and cross fingers,
e.g.,

  ;; Enable in prog mode buffers.
  (add-hook 'prog-mode-hook #'bug-reference-prog-mode)

and (for Gnus users)

  ;; Enable in Gnus summary and article buffers.
  (add-hook 'gnus-summary-mode-hook #'bug-reference-mode)
  (add-hook 'gnus-article-mode-hook #'bug-reference-mode)

Bye,
Tassilo