[Emacs-orgmode] configurable link export

[Austin Frank]

First off, thank you to Carsten and David for org and org-publish. 
These tools have made it easier to get myself organized locally and 
online than it ever has been before.

I'd like to suggest a configuration option that influences the way links 
are exported in the org-export-as-* functions.

When I create local org files, I link to whatever files on my disk are 
relevant and useful.  When I publish those org files, some of the org 
links (like to other published org files) still work and make sense as 
html links.  Others (like links to local documents or directories) don't 
make any sense when published-- the resources they pointed to on the 
local system aren't on the remote filesystem that hosts the html pages.

To be more specific, I maintain a directory ~/notes/ and a directory 
~/blog/, each with a bunch of org files underneath it.  I then use 
org-publish to generate the html files, which are then uploaded to my 
website.  Crucially, this same directory structure is mirrored at my 
website, so local org links between those files work perfectly as html 
links when they are published and uploaded.

I'd love to be able to specify that I want all org links of 
[file:///home/aufrank/notes/*] and [file:///home/aufrank/blog/*] to be 
exported as full html links, and exclude org links to any other files on 
my filesystem at export time.

I think that David has established a good system for including and 
excluding files during publishing.  Org-publish first generates a list 
of files to publish based on regex matching of the extensions of files 
in a directory.  These are filtered through a regex-based exclude list, 
and then individual files can be added back in with an include list.

Applying this strategy to link export, I might want to have something 
like the following in my org-config.el:

(setq org-export-links-extension "org\\|txt"
      org-export-links-exclude   "~/*"
      org-export-links-include   ("~/notes/", "~/blog/"))

This would result in exporting all of the .org and .txt files in 
~/notes/ and ~/blog/ as working links in html files, but would ignore 
links in org files to anything else in my home directory during export.

There's certainly a question about how org links that are not exported 
as html links should be handled.  I would actually be fine with just 
exporting the double-bracketed notation, but I bet there are better 
ideas out there.

Thanks again for the org suite,
/au