Icongenie: Relative padding

Is your feature request related to a problem? Please describe. The padding option adds a fixed number of pixels padding regardless of the size of the image being output, so adding 10px padding to a 30x30 image would end up with a tiny 10x10 image in the middle, while with a 1000x1000 image the padding would hardly be noticeable.

Describe the solution you'd like Specify the padding as a percentage of the image height and/or width

Describe alternatives you've considered I've had to add padding manually to my source image and use the --skip-trim option