[Orgmode] Feature request: custom link handlers

[Tom Weissmann]

This is basically a request to be able to create custom link handlers. 

For example, my `org-link-abbrev-alist' is this:

`(("wiki"   . org-link-abbrev-wiki)
  ("bmk"    . org-link-abbrev-bookmark)
  ("POA"    . org-link-abbrev-poa))


Only one of these actually returns a link that Org mode can use: wiki returns a
url. "bmk" opens a bookmark and "POA" opens another application; neither return
anything, so using them raises an error message.

One possibility would be to allow the link abbreviations to return two values:
nil and a message. `org-link-abbrev-poa' could then return 
'(nil 
  "Opening POA XYZ")
and Org mode wouldn't fret about not receiving a null link, but simply display
the message.

Another possibility would be to create a door to add custom link handlers with
custom completion - that way one could add bookmark links with minibuffer
completion...