Adding tvOS support

[bkbeachlabs]

Thanks for this tool! I think it would be great if it also added tvOS support. I spent some time looking around last week and couldn't find anything that does that yet.

I know the images for tvOS are a bit of a challenge since they aren't all square, but you could do a fairly simple scale to fit from the basic square icon, and then just pad the width with empty space or whatever the colour at the edge of the icon is to get started.

Thanks!