Add support for SnapKit

[moto0000]

Add support for SnapKit https://github.com/SnapKit/SnapKit

[KingOfBrian]

Adding constraint libraries should work easily, by adding another instance of a ConstraintCodeGenerator, but the code generation is still pretty primitive. This will need some structural enhancement to support the operations inside a block. It's probably a pretty simple enhancement, but let me think on how it should look for a few.