Feed displays should have Link display override option

Feeds should have a link display override option so if multiple feeds exist in a single view, each one can have its
tag point to a different place, rather than them all being forced into using the default. The form is disabled because feed displays have a path and the default display plugin ends up interpreting this to mean it's like a page and doesn't have a link. The patch to fix this is very simple and attached, involving overriding the get function in the feed display class.

Thanks,
David