Add configure block to allow input customisation

[Sherlouk]

The idea of the library is to allow users to stitch multiple variants of the same value into a single image. This is possible using the different traits (such as dynamic text size or user interface style) or image strategies (such as device size).

However, it doesn't allow you to apply customisations to the view which are done at a custom class level. To be clearer, we should add the ability to change variables on the view itself for each test in the stitch.

assertSnapshot(matching: vc, as: .stitch(strategies: [
  ("iPhone 8", .image(on: .iPhone8), configure: { $0.theme = .light }),
  ("iPhone 8 Plus", .image(on: .iPhone8Plus), configure: { $0.theme = .dark }),
]))