Adds customization of cropper color and limit black overlay to image only

[faltakam]

• Original color of cropper color boundaries is set as default value.
• Black overlay stripes on sides, which appear when image aspect ration doesn't match canvas aspect ratio, can be confusing when image background color is the same as background color of canvas parent element. In that case image borders aren't visible and stripes that fill canvas appear to be part of the image. When resizing or moving crop you are restricted to image only but due to invisible image borders it appears that some part of image is inaccessible. Limiting overlay to image only fixes the issue.

[micros123]

Useful feature, code looking good, hope Allan can merge this soon?

[alinearielo]

+1 :D