Add autoload cookies, reformat for packaging

(In connection with https://github.com/milkypostman/melpa/pull/1080)

* shorten package description
* autoload the mode definition
* follow conventions for number of `;;` in comments
* avoid setq-local, which is only available in latest HEAD emacs
* use a consistent namespace prefix for identifiers


Note also, though, that `try-to-rebind` is referenced here, but doesn't seem to be available in Emacs, so I think that something's wrong...

It also occurred to me that you might want to split the mode into a regular minor mode, and a globalized version of it. That way, users can more easily enable it in individual buffers/modes. An example of this scheme would be https://github.com/purcell/whitespace-cleanup-mode.

P.S. checkdoc-mode will help you check that suitable docstrings are in place.