Feature request: multiple generate multiple sizes of the icon
It would be cool to have an ability to generate multiple sizes of the icon. Currently plugin allows only to create only one. 

![image](https://user-images.githubusercontent.com/18515722/35182580-91c18ca8-fde0-11e7-9a0d-f2697e046926.png)

For example, allow multiple fields for size or maybe a dynamic button “+ add size”.
