Open extension points for other major modes

To create support for other major modes than sgml-mode, there are four
functions that need to be created and declared for that mode.

- te/skip-tag-forward-fn
- te/skip-tag-backward-fn
- te/empty-tag-p-fn
- te/current-tag-fn

They should be locally overridden, for instance

(add-hook 'web-mode #'(lambda ()
  (set (make-local-variable 'te/skip-tag-forward-fn)
       'te-web-mode/skip-tag-forward)
  ;; etc)

See each function for documentation of its behavior.