Handle centered images better.

* Disable left and right margin inputs when an image is centered
* Remove any existing left and right margin inputs after centering
Use html5 color input if supported by the browser
Adjust the layout.

Change order to top, bottom, left, right.
Remove (px).
Adjust behavior to distinguish an unset value from zero.
Bump version and update changelog.
Visual tweak to improve display on small screens.
Add 0.2 stable tag.
Add a release script.