Preloads this link when the user hovers over it.
When the link is clicked later the response will already be cached,
making the interaction feel instant.
Preloading a link will not abort pending requests
targeting the same fragments. Only when the link is clicked later
conflicting requests are aborted.
Preload requests are considered background requests
and will not show the progress bar.
Preloading a link will enable caching for that link automatically.
Unpoly will only preload links with safe methods.
