Implement <link> and <a rel> prev/next (etc.) motions

Implement <link> and <a rel> prev/next (etc.) motions

* eww.el (eww-next-url, eww-previous-url, eww-up-url, eww-top-url):
New defvars.
(eww-open-file): New defun.
(eww-render): Initialize new variables.
(eww-display-html): Handle "link" and "a".
(eww-handle-link, eww-tag-link, eww-tag-a): New defuns.
(eww-mode-map): Move "p" to "l".  Bind "p", "n", "t", and "u".
(eww-back-url): Rename from eww-previous-url.
(eww-next-url, eww-previous-url, eww-up-url, eww-top-url): New
defuns.