Adding link_attrs to column.

Add the ability to attach attributes to a column containing a link via
the link_attrs property.

The new link_attributes parameter will eventually replace the less
general link_class attribute.

Co-Authored-By: Davide Guerri <davide.guerri@hp.com>
Change-Id: Iea79aec970a2fe8711a9f5a8b973407df88dcdec
Implements: blueprint add-attribute-links
Closes-Bug: #1298533
Merge "Adding link_attrs to column."