Allow regexps in org-file-apps to capture link parameters using groups

Allow regexps in org-file-apps to capture link parameters using groups

Patch by Jan Böker.

Jan writes:

> What is this?
> =============
>
> This patch changes the way extension regexps in `org-file-apps' are
> handled. Instead of against the file name, the regexps are now matched
> against the whole link, and you can use grouping to extract link
> parameters which you can then use in a command string to be executed.
>
> For example, to allow linking to PDF files using the syntax
> file:/doc.pdf::<page number>, you can add the following entry to
> org-file-apps:
>
> Extension: \.pdf::\([0-9]+\)\'
> Command:   evince "%s" -p %1
>
> In a command string to be executed, the parameters can be referenced
> using %1, %2, etc.  Lisp forms can access them using (string-match n link).
>
>
> Where to get it?
> ================
> Either apply the patch by hand or
>
> git pull git://github.com/jboecker/org-mode.git org-file-apps-parameters
>
>
> What's next? / Feedback
> =======================
>
> - Find the bugs. Since this messes with links, a central concept of Org,
> I probably have missed some edge cases; so please test this and
> report if it works for you.
>
> I also appreciate any feedback on code quality or the design decisions
> made. I am learning elisp along the way, so you may be able to write
> some changes in a more idiomatic and/or elegant way.
>
> - Add a mechanism for org-mode modules to add default values to
> org-file-apps, similar to the variables org-file-apps-defaults-*.
> This could be used by modules to define their own extensions to the
> syntax of file: links.
>
> - Modify org-docview.el to use this and deprecate the docview: link syntax.
>
>
> What does it (intentionally) break?
> ===================================
>
> This patch introduces a backwards-incompatible change. If LINE or SEARCH
> is given, the file is no longer guaranteed to open in emacs: if IN-EMACS
> is nil and an entry in org-file-apps matches, that takes precedence.
>
> A grep of the lisp/ and contrib/ directories showed that no code in the
> org-mode distribution was relying on this behaviour; whereever LINE or
> SEARCH is given, IN-EMACS is also set to t.
>
> I decided against adding an additional parameter because that would be
> redundant; the original link as seen by org-open-at-point can be
> reconstructed from PATH, LINE and SEARCH.
>
> I am not that sure if this is the right way to do this, but it seems to
> break as little as possible while hopefully avoiding to add too much
> complexity.