Introduce function to create links to attachments.

Introduce function to create links to attachments.

This patch introduces two new functions in org-attach.el,
`org-attach-expand', and `org-attach-expand-link'.  Both take a file
name as an argument, assume that this file is an attachment of the
current entry, and return the full path to this file or a
"file:..." link to it, respectively.

With these functions, it becomes very easy to use link abbreviations
to create a new link type to attachments:

After

(setq org-link-abbrev-alist '(("att" . org-attach-expand-link)))

links like [[att:file.txt]] will work.