Integrate some comments from Rasmus

See http://permalink.gmane.org/gmane.emacs.devel/179381 for the
discussion.

* darkroom.el: Add file header.
(darkroom): Add customization group.
(darkroom-margins): Make it a `defcustom'.
(darkroom-turns-on-visual-line-mode): ditto.
(darkroom-mode): Improve docstring.
(darkroom-fill-paragraph-maybe): Removed.
(darkroom-visual-mode-maybe-enable): Removed.
(darkroom-mode): Improve docstring.
(darkroom-mode): Use `darkroom-text-scale-increase'
(darkroom-maybe-enable): Comment out debug code.
(darkroom-mode): Don't `darkroom-visual-mode-maybe-enable'
(darkroom-mode-map): Don't bind `darkroom-fill-paragraph-maybe'.
(darkroom-saved-visual-line-mode): Removed.
(darkroom-text-scale-increase): New defcustom.
Integrate another comment from Rasmus

* darkroom.el (darkroom-margin-increment): New defcustom.
(darkroom-increase-margins): Take a parameter.
(darkroom-decrease-margins): Use `darkroom-increase-margins'
Tidy up margin calculation

* darkroom.el (darkroom-compute-margins): Renamed from `darkroom-margins'.
(darkroom-buffer-margins): New variable.
(darkroom-set-margins): Redesign. Set `darkroom-buffer-margins'.
(darkroom-increase-margins): Redesign.
(darkroom-mode-map): Fix whitespace.
(darkroom-mode): Use `set-mark-local-variable'
(darkroom-mode): Save and restore current margins.
(darkroom-tentative-mode): Fix whitespace.
Add (still non-functioning) `darkroom-compute-margins'

* darkroom.el: Require 'cl-lib
(darkroom-guess-margins): New function for putting in
`darkroom-margins'. But broken due to `window-width' not being
suitable probably.
(darkroom-compute-margins): Redesigned.
(darkroom-set-margins): Don't `darkroom-compute-margins'
(darkroom-mode): Reordered statements.