Add "exists" tag
exists
Is a special tag without a validation function attached. It is used when a field
is a Pointer, Interface or Invalid and you wish to validate that it exists.
Example: want to ensure a bool exists if you define the bool as a pointer and
use exists it will ensure there is a value; couldn't use required as it would
fail when the bool was false. exists will fail is the value is a Pointer, Interface
or Invalid and is nil. (Usage: exists)
see example in issue #142 thanks @davisford