Separate validation from unmarshaling code

[LarsStegman]

Hi,

we are working on a tool that reads a YAML file and then does some transformations on the ASTs. We would then like to decode the transformed AST into the generated objects. Because we decode the AST directly instead of unmarshalling a YAML file, we cannot use the validation code. We would like to separate the validation code from the Unmarshal functions. Are you open to us making a pull request at some point? I am not yet sure about the time frame for this, though.