Indicates if this paricular plugin can (and want) to render the given search result.
Return true if and only if the plugin can render the given item without throwing exceptions.
Note that several plugins can return true for the same SearchResult in which case
one will be picked in an undefined order.
Render the given search result. A display ready html string should be returned.
If rendering fails (maybe the attachment to render was removed between the call
to canRender and render), the plugin should return null to indicate that it failed
to render the result