Allow multiline structure tags (ignore all the types of whitespace).

Hello. I'm quite surprised that there is no such feature have been discussed yet. I prefer to break struct tags into lines, but the multiTag scanner allows only `' '` as a whitespace.

The commit makes things like this possible:
```go
type Options struct {
	Port int `short:"p" long:"port" default:"5051"
	description:"Specify a port to bind the web interface"`
}
```

(you know, the long-description may get even longer)