add len,min,max,regex... support for pointer type

[YueHonghui]

In our project, we use struct contains pointer type variables to distinct between empty and nil value. But validator not supported pointer type couldn't satisfy this situation. So I implement len,min,max,regex... builtin validators for elem of pointer types.

Request to merge.

[jameshartig]

I'm not sure what we should do about the nil problem. Off the top of my head: length should be 0 for nil, and maybe the same with min/max and then for regex maybe don't return an error. Either way we should document in the function doc what happens with nil pointers. I would also be fine saying that they're not handled (like you have now) and you have to use nonzero if you don't want to accept nil.

[YueHonghui]

Yes, you are right. In this PR, I just omit the nil pointer, leave it to the nonzero validator. The validators separated by , in tags, actually represents expression validator1 and validator2 and validator3. If validators other than nonzero can handle nil as False, we would have no way to represent expression zero or validator. I will update the PR with a better doc to this.