[PROPOSAL] Use prefix arg to control scope of org-narrow-to-subtree.

[Karl Fogel <kfogel@red-bean.com>]

Hi.  This is a feature proposal -- if the consensus is that it would be welcomed, I'm happy to code it.  I just didn't want to take the time to write it if there's no chance for it to be accepted upstream (since I don't want to be maintaining my own personal branch of Org Mode).

It would be useful if `org-narrow-to-subtree' could optionally narrow to the next subtree(s) up, rather than only to the subtree point is currently in.  For example, assume this text:

   * This is the first level
   Some text here.
   ** This is the second level
   Some other text here.
   *** This is the third level
   By now we all know this song.
   It is such a pretty song.
   **** This is the fourth level
   But do we have to sing it all day long?
   This car trip is getting incong
   ***** This is the fifth level
   ruously unrhymed.

Further assume that point is on the "c" of "car trip".  

In the current Org Mode, if you type `C-x n s', it will narrow to the fourth-level subtree (with the fifth level included in the narrowed buffer, of course).

Since `org-narrow-to-subtree' takes no arguments at all right now, it's conveniently ripe for improvement :-).

My proposal is for each raw prefix arg (each `C-u' prefix) to expand the narrowing level outward/upward by one.  So in the above situation:

  - `C-u C-x n s' would narrow to the third-level subtree

  - `C-u C-u C-x n s' would narrow to the second-level subtree

And so on.

If you offer too many `C-u's, such that the narrowing would be wider than the current surrounding first-level subtree, then there are two possible ways we could handle it:

1) Extra `C-u's are ignored -- just narrow to surrounding 1st-level subtree.

2) Throw an error.

I prefer (1), because it would be the more useful behavior, even though (2) would be easier to implement (since `org-back-to-heading' already throws the error).  However, I'd welcome others' feedback on that question, or on any other aspect of this proposal.

Best regards,
-Karl