0.7.0: Custom validations

• Enhances custom validations by adding validate and overloading required and ifPresent to transform data before doing the validation, see the "custom validations" section of the README (#65 by @NikkyAI and @dhoepelman)
• Adds support for adding validations on zero-argument functions, which is especially useful for Java getters (#65 by @NikkyAI)