Links: Implement special completion support for some link types
The commit implements a general mechanism for providing special
(e.g. completion) support for specific link types when entering links
with `C-c C-l'. After calling `C-c C-l', you may now press RET after
inserting a link prefix, and Org will look for a function
`org-PREFIX-complete-link'. Such functions may be defined for any
link types, including link abbreviations. Currently, Org has
`org-file-complete-link' for file name completion, and
`org-bbdb-complete-link' for completion of record names from BBDB.