What's new or fixed?

• Added new issn validator. TY @mrcook for the PR
• Added improvement/fix to the file url validation. TY @nodivbyzero for the PR
• Fix onof tag in examples. TY @gren236 for the PR
• Add fmt.Stringer interface support to uuid validations allowing most UUID validation libraries which implement it to work transparently now. TY @JoshGlazebrook for the PR
• Added new omitnil similar to omitempty but for pointers. this is mainly for code generation ease. TY @tarampampam for the PR