Link attributes not working on numeric reference links

[nrcaldwell]

Numeric link references work, but if you add a link attribute it is not included in the output. Implicitly named link references have this same bug.

Example input :

This is a footnote style [reference link][1] with an attribute. 

[1]: http://domain.com "reference" target=_blank

Expected output:
<p>This is a footnote style <a href="http://domain.com" title="reference" target="_blank">reference link</a> with an attribute.</p>

Actual output:
<p>This is a footnote style <a href="http://domain.com" title="reference">reference link</a> with an attribute.</p>