add extended attributes for :target links

fixes #2847.  This PR adds OS X-specific extended attributes
(metadata) whenever :target is used on an artifact.  The
extended attributes are available for search software such
as Spotlight or Alfred to discover the artifact under the
:target name.

Limitations: Spotlight specifically chooses to ignore this
attribute for App bundles.  Alfred will also ignore this
attribute until the upcoming 2.2 release.
Merge pull request #3073 from rolandwalker/add_target_metadata

add extended attributes for `:target` links