Add new tag "structonly"

[deankarn]

a flag is needed to identify that validation should run for a struct i.e. struct is required but not validate it's containing fields.

example:
type Inner struct{
Test string validate:"len=5"
}

type Outer struct{
InnerStruct Inner validate:"required,structonly"
}

in the above example InnerStruct will validate on the required, but Inner's Test field will not be validated.