Org reader: absolute, relative paths in links

The org reader was to restrictive when parsing links, some relative
links and links to files given as absolute paths were not recognized
correctly.  The org reader's link parsing function was amended to handle
such cases properly.

This fixes #1741
Org reader: allow empty links for gitit interop

While empty links are not allowed in Emacs org-mode,  Pandoc org-mode
should support them: gitit relies on empty links as they are used to
create wiki links.

Fixes jgm/gitit#471
Merge pull request #1742 from tarleb/org-fix-links

Org fix links