org-attach.el: Get attachments from git annex

org-attach.el: Get attachments from git annex

* org-attach.el (org-attach-use-annex): New function to check if git
  annex should be used.
  (org-attach-annex-get-maybe): New function to get a file from git
  annex if necessary.
  (org-attach-annex-auto-get): New defcustom to determine behavior
  of org-attach-annex-get-maybe.
  (org-attach-open): Automatically get attached files from git annex when
  opening if necessary.
* testing/lisp/test-org-annex.el: New file for testing org-attach. Only
  contains code for testing org-attach with git annex at the moment.
* mk/targets.mk: Fix cleantest target so it can delete git annex repos.