Add require field tag

[NilsJPWerner]

It would be fantastic if this library had the option to set a struct field as required. Then when defaults.Set is called on a struct with zero value fields that are marked as required the function returns an error.

[superstes]

FYI: Had to write a library for validating structs that can handle 'required'
https://github.com/O-X-L/go-validator