Feature/background colors

Add controls to be able to alter the color of the transparent background.

![Animated gif of this](http://cl.ly/image/030I3W3M3V39/output.gif)

Next, I'd love to add a “custom” button next to the two that detects the `color-picker` plugin, 
and opens it to let it select a color – but maybe you've got some reservations when it comes
to plugins “talking” to each other. 

What do you think? :)
Also, more importantly, why is this check needed for jpegs?