Add coverage with Snapshot Tests

[dzenbot]

Since this library is all about rendering, to make sure things don't break after v1, we should add as many useful snapshot tests as possible. Things to cover:

• [x] Icon conversion to image
• [x] Icon conversion to image with padding (both, vertical and horizontal)
• [x] Icon conversion to attributed string
• [x] Composition of text + icon attributed string
• [x] UIKit extensions