Note that image links are now supported

Through an accident of good fortune, rdoc-image: also supports image
links.  Since the image needs to be fully qualified, the support is not
very good.

Fixes #93
Make hard-linking more robust

For the first pass I didn't try building RDoc twice with the same output
directory.  With the original code this would fail as the hard-link
already existed.

Now the original file is removed and hard-linking is attempted a second
time when the destination exists.

See #186