Extending unique to ensure uniqueness of a specific field in a slices of struct

[shihanng]

Fixes Or Enhances # .

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

Change Details:

Hi, wonder if you think it makes sense to include this in the package:

Let's say we have a struct like the following,

type People struct {
	Name string
	Email string
}

which will be constructed into []People{....}. And we want to check if all the values in the Name fields are unique. This PR allows us to do that by extending the unique functionality: unique=Name.

Thank you for taking your time to review this PR ❤️

@go-playground/admins

[coveralls]

Coverage remained the same at 100.0% when pulling 8efe088 on shihanng:unique_struct_field into 556b9da on go-playground:v9.