structure: stop parsing nested structures via field tag

This is a new feature that is going to let us enable stopping parsing nested structures with the tag option "omitnested" if it's provided. An example structure would be : 

```go
type A struct {
	Name  string
	Value string
	Time  time.Time `structure:",omitnested"`
}
```

Here if used with the `Map()` function, the `Time` field will not recursively parsed and the value is going to used directly.