Add support for negated boolean flags

Will automatically set a boolean Struct field to false if the flag starts with `--no-`, even when the default is `true`.

For example:
```
type Cmd struct {
  Output bool `default:"true"`
}
```

Calling
```
command
```
Will set `Output` to `true`, but
```
command --no-output
```
Will set `Output` to `false`.

model.go

@@ -227,6 +227,7 @@ type Value struct {
 	Tag          *Tag
 	Target       reflect.Value
 	Required     bool
+	Negated      bool
 	Set          bool   // Set to true when this value is set through some mechanism.
 	Format       string // Formatting directive, if applicable.
 	Position     int    // Position (for positional arguments).