Org reader: properly handle links to `file:target`

Org links like `[[file:target][title]]` were not handled correctly,
parsing the link target verbatim.  The org reader is changed such that
the leading `file:` is dropped from the link target.

This is related to issues #756 and #1812.
Merge pull request #1813 from tarleb/file-links

Org reader: properly handle links to `file:target`