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`.

context.go

@@ -579,6 +579,9 @@ func (c *Context) getValue(value *Value) reflect.Value {
 		}
 		c.values[value] = v
 	}
+	if value.Flag != nil && value.Flag.Negated {
+		v.SetBool(false)
+	}
 	return v
 }
 
@@ -639,15 +642,19 @@ func (c *Context) parseFlag(flags []*Flag, match string) (err error) {
 	for _, flag := range flags {
 		long := "--" + flag.Name
 		short := "-" + string(flag.Short)
+		neg := "--no-" + flag.Name
 		candidates = append(candidates, long)
 		if flag.Short != 0 {
 			candidates = append(candidates, short)
 		}
-		if short != match && long != match {
+		if short != match && long != match && neg != match {
 			continue
 		}
 		// Found a matching flag.
 		c.scan.Pop()
+		if flag.Value.IsBool() && match == neg {
+			flag.Negated = true
+		}
 		err := flag.Parse(c.scan, c.getValue(flag.Value))
 		if err != nil {
 			if e, ok := errors.Cause(err).(*expectedError); ok && e.token.InferredType().IsAny(FlagToken, ShortFlagToken) {

kong_test.go

@@ -356,6 +356,19 @@ func TestTraceErrorPartiallySucceeds(t *testing.T) {
 	require.Equal(t, "one", ctx.Command())
 }
 
+func TestNegatedBooleanFlag(t *testing.T) {
+	var cli struct {
+		Cmd struct {
+			Flag bool `kong:"default='true'"`
+		} `kong:"cmd"`
+	}
+
+	p := mustNew(t, &cli)
+	_, err := p.Parse([]string{"cmd", "--no-flag"})
+	require.NoError(t, err)
+	require.Equal(t, false, cli.Cmd.Flag)
+}
+
 type hookContext struct {
 	cmd    bool
 	values []string

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).