Support separate border padding / shape padding

[johnnotron]

Would be useful to have additional borderPadding setting, as well as just padding and extrude, so we can implement decreasing padding around the edge of a sheet to save space / allow max width or height items to fit into a sheet.

In TexturePacker I can set border padding 0 and shape padding to 4 for example. This results in padding between items, but the padding on items against edge of sheet being removed to save space.

Happy to have a go a making a PR for this, if someone can point me the direction to get started 🙏